Holographic fingerprint

ABSTRACT

A holographic projector comprising a spatial light modulator arranged to display a hologram of a light pattern for projection and to spatially-modulate light, in accordance with display, to form a holographic reconstruction, wherein the holographic reconstruction is spatially-separated from the spatial light modulator. If the holographic projection is operating properly, the formed holographic reconstruction should correspond to the light pattern. The holographic projector also comprises a detector array comprising a plurality of light detection elements arranged to detect light corresponding to a respective plurality of positions of the holographic reconstruction and to provide a respective plurality of output signals related to light detection, and a fault detection circuit arranged to compare one or more of the plurality of output signals from the respective plurality of light detection elements with one or more of a plurality of expected signals based on the light distribution of the light pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a non-provisional patent application claimingpriority to United Kingdom Patent Application No. GB 2012165.3, filedAug. 5, 2020, the contents of which are hereby incorporated by referencein their entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a projector. More specifically, thepresent disclosure relates to a holographic projector, a method ofholographic projection and holographic projection system. Embodiments ofthe present disclosure relates to a light detection and ranging,“LIDAR”, system arranged to make time of flight measurements of a sceneand to a method of monitoring operation of a LIDAR system, for examplefor safety monitoring. Some embodiments relate to an automotive LIDARsystem or to a LIDAR system comprised within a portable device. Otherembodiments relate to a head-up display having improved safety.

BACKGROUND

Light scattered from an object contains both amplitude and phaseinformation. This amplitude and phase information can be captured on,for example, a photosensitive plate by well-known interferencetechniques to form a holographic recording, or “hologram”, comprisinginterference fringes. The hologram may be reconstructed by illuminationwith suitable light to form a two-dimensional or three-dimensionalholographic reconstruction, or replay image, representative of theoriginal object.

Computer-generated holography may numerically simulate the interferenceprocess. A computer-generated hologram may be calculated by a techniquebased on a mathematical transformation such as a Fresnel or Fouriertransform. These types of holograms may be referred to asFresnel/Fourier transform holograms or simply Fresnel/Fourier holograms.A Fourier hologram may be considered a Fourier domain/planerepresentation of the object or a frequency domain/plane representationof the object. A computer-generated hologram may also be calculated bycoherent ray tracing or a point cloud technique, for example.

A computer-generated hologram may be encoded on a spatial lightmodulator arranged to modulate the amplitude and/or phase of incidentlight. Light modulation may be achieved using electrically-addressableliquid crystals, optically-addressable liquid crystals or micro-mirrors,for example.

A spatial light modulator typically comprises a plurality ofindividually-addressable pixels which may also be referred to as cellsor elements. The light modulation scheme may be binary, multilevel orcontinuous. Alternatively, the device may be continuous (i.e. is notcomprised of pixels) and light modulation may therefore be continuousacross the device. The spatial light modulator may be reflective meaningthat modulated light is output in reflection. The spatial lightmodulator may equally be transmissive meaning that modulated light isoutput in transmission.

A holographic projector may be provided using the system describedherein. Such projectors have found application in head-up displays,“HUD”, and head-mounted displays, “HMD”, including near-eye devices, forexample. The holographic projector may be used for light detection andranging (LIDAR). Light detection and ranging (LIDAR) systems may be usedin a variety of applications including portable devices and vehicles.

A moving diffuser may be used to improve image quality in devices whichuse coherent light such as holographic projectors.

The present disclosure is concerned with improvements in monitoringoperation of holographic projectors, for example within light detectionand ranging (LIDAR) systems. In particular, such improvements mayinclude more reliable and/or more accurate techniques for monitoringsafe operation of a hologram display device, within a LIDAR system.

SUMMARY

Aspects of the present disclosure are defined in the appendedindependent claims.

In general terms; a method, system and apparatus are provided, whichenable accurate and efficient monitoring and control of the operation ofa holographic projector, for example a holographic projector comprisedwithin a light detection and ranging, “LIDAR”, system arranged to maketime of flight measurements of a scene. The monitoring and control canensure safe operation of the holographic projector and of a light sourcearranged to illuminate the holographic projector. One or more lightdetectors are provided to monitor light signals. The monitored lightsignals may be from a holographic reconstruction, formed when a displaydevice such as a spatial light modulator (SLM), displaying a hologram ofa light pattern, is irradiated by suitable light, such as laser light.In another example, the monitored light signals may be from light thatis travelling from an irradiated SLM, towards a holographic replayplane, and thus has not yet fully formed a holographic reconstruction.The monitored light in such an example may be referred to as being, oras being comprised within, a ‘partial’ holographic reconstruction. Inanother example, the monitored light signals may be from light that hasformed a holographic reconstruction, which may be referred to as an‘intermediate’ holographic reconstruction, on a holographic replay planeand is being projected towards a screen, diffuser or other plane, toform an image of the intermediate holographic reconstruction thereon.The intermediate holographic reconstruction may itself be formed in freespace or it may, for example, be formed on a screen, such as a diffuser.

The detectors comprise, or are communicatively coupled to, a processoror fault detection circuit that is configured to compare a light signalthat is detected by the detectors, for a particular hologram at a giventime, to an expectation of a light signal that should have beendetected, for that particular hologram at that time, if the holographicprojector was operating accurately and safely. In particular, the methodcan determine whether the display device is correctly displaying thedesired hologram, at a given time, based on the light signals from theholographic reconstruction and/or whether the light is at an eye-safelevel of brightness or intensity. The method may further comprisecontrolling the light source to pause or stop irradiating (i.e.illuminating) the display device, or to change a parameter of theirradiating light, if there is an indication that it may not befunctioning correctly. This can therefore act as a safeguard against eyedamage and/or eye discomfort for an observer, which might otherwisearise if the display device was allowed to continue inaccurate orimproper operation. In some cases, the method may comprise controllingthe light source to reduce the intensity of irradiation. This couldensure eye safety while continuing to provide enough light to monitor todetermine if/when the SLM (or other aspect of the projector) hasrecovered from its error state.

According to an aspect, a holographic projector is provided, comprisinga spatial light modulator (SLM) arranged to display a hologram of alight pattern for projection and to spatially-modulate light, inaccordance with display, to form a holographic reconstruction, whereinthe holographic reconstruction is spatially-separated from the spatiallight modulator. If the holographic projection is operating properly,the formed holographic reconstruction should correspond to the lightpattern. The holographic projector also comprises a detector arraycomprising a plurality of light detection elements arranged to detectlight corresponding to a respective plurality of positions of theholographic reconstruction and to provide a respective plurality ofoutput signals related to light detection, and a fault detection circuitarranged to compare one or more of the plurality of output signals fromthe respective plurality of light detection elements with one or more ofa plurality of expected signals based on the light distribution of thelight pattern.

The fault detection circuit—or another processor or circuit that iscomprised within or communicatively coupled to the holographicprojector—may be arranged to determine, as a result of said comparison,whether a difference exists between the one or more of a plurality ofoutput signals and the one or more of a plurality of expected signals.

The purpose of the comparison, made by the fault detection circuit, maybe to assess the validity of the holographic reconstruction. Thecomparison may determine whether the holographic projector is operatingsafely, or whether there is a risk that it may not be. The comparisonmay compare an output signal from each of a plurality of light detectionelements to an expected output signal from each of those light detectionelements, at a given time or times. One or more of the expected timesignals may be time-varying. For example, the time variation of theexpected signals may be due to an expectation that the hologram willchange, thereby changing the light pattern, and/or to an expectationthat the light pattern will dynamically change position or location,thereby changing which part (if any) of the light pattern would beexpected to occur at the location of a particular light detectionelement, at a given time or times. For example, the holographicreconstruction may be expected to be translated or “scanned” across areplay plane. In some cases, the identity of the detection element/s forwhich a light signal/s is expected may change, over time.

The plurality of output signals from the respective plurality of lightdetection elements (and/or the plurality of expected signals) maycomprise a combined or concatenated signal, from the plurality (or asubgroup or subset of the plurality) of light detection elements in thedetector array. For example, the light detection elements may beconfigured to provide binary signals, indicating the presence (‘1’) orabsence (‘0’) of a light signal, at any given time. A concatenatedsignal may comprise a sequence of the binary outputs from each of aplurality of light detection elements (or a sequence of the expectedbinary outputs from those light detection elements.) The length, inbits, of such a concatenated binary signal may be equal to the number oflight detection elements to which it relates. In another example, thelight detection elements may be configured to provide non-binary(“greyscale”) signals, which provide information on, for example, theintensity or brightness of the detected light.

The holographic projector may comprise, or be provided in conjunctionwith, a light source. The light source may be a laser light source. Thelight may be, for example, infra-red (IR) light, visible light orultra-violet light.

The fault detection circuit may comprise, or be comprised within orcommunicatively coupled to, any suitable controller or processor. Thatcontroller or processor may also be configured to perform other actions,in relation to the holographic projector. For example, it may bearranged to control the selection and display of holograms on thespatial light modulator. The fault detection circuit may be referred tosimply as a ‘controller’ or as a ‘signal comparison circuit’ or as anyother appropriate term.

The fault detection circuit may be arranged to alter or to preventfurther light projection, if it identifies a difference between said oneor more output signals from the respective plurality of detectionelements and the one or more expected signals, to ensure safe operationof the holographic projector. In some cases, it may be arranged to notentirely prevent further light projection, but to alter the lightprojection, for example by reducing its intensity. For example, theholographic projector may be arranged to prevent or reduce further lightprojection from a light source towards the spatial light modulator(SLM), and/or to prevent or reduce light being emitted by the SLM,and/or to prevent or reduce light that is emitted by the SLM fromreaching its target object or scene, and/or to prevent or reduce lightfrom being reflected from that scene or object, towards an observer, ifit identifies a difference between said one or more output signals fromthe respective plurality of detection elements and the one or moreexpected signals. For example, the operation of the light source may bepaused or dialed-down and/or the operation of the SLM could be paused ornullified, for example via the activation of a shutter or other barrier,located either between the light source and the SLM and/or between theSLM and a scene or target that it is otherwise arranged to illuminate,or even between the scene or target and the observer. In some cases,multiple barriers or shutters may be employed, for blocking the lightpath at multiple different respective locations or positions, betweenthe light source and the observer.

The fault detection circuit may be arranged to tolerate certaindifferences, between one or more output signals from the respectiveplurality of detection elements and the one or more expected signals.For example, a difference may be tolerated (and, thus, further lightprojection allowed) if that difference is of a value (i.e. of amagnitude, or extent) that is not greater than an acceptability value.In other words; the fault detection circuit may allow certaindifferences to exist, up to a predetermined threshold, but may determinethat differences that are of a value that exceeds that predeterminedthreshold are not acceptable, and so the further light projection mustbe stopped, reduced or paused, until the cause of those differences isinvestigated and, if appropriate, remedied. The fault detection circuitmay be arranged to distinguish between differences of differentrespective types or natures. There may be some differences (ormis-matches) between a received signal and an expected signal that aredeemed to be unacceptable, regardless of their magnitude, whereas otherdifferences (or mis-matches) between a received signal and an expectedsignal may be deemed to be acceptable, and to present a low safety risk,if their magnitude is relatively small. In some cases, certaindifferences (or mis-matches) between a received signal and an expectedsignal may prompt the controller to reduce the intensity of the lightbut not to prevent it entirely, at least for a predetermined window oftime, thus giving an opportunity for the source of the difference to beaddressed and remedied, without the need to stop operation of theprojector entirely during that time window.

In some arrangements, the type and/or the extent of a detecteddifference may determine (or contribute to a determination as to) whichstep or steps is/are taken, to block the light path, between the lightsource and the observer.

The holographic reconstruction of the light pattern, which a hologramrepresents, may be formed on a holographic replay plane in free space oron a screen such as a diffuser or in the eye of an observer. Theholographic replay plane may be planar. In some cases, the holographicreplay field may not be planar. For example, different spots within aholographic reconstruction may come into focus at different respectivedepths within the same three-dimensional (3D) image.

The light pattern that is represented by the hologram may betime-varying, such that one or more of the plurality of expected signalsmay also be time-varying. For example, the identity and/or the locationof the light detection element or elements, within the detector array,that are expected to output a signal, related to light detection, mayvary with time.

A sequence of light patterns may be represented by a correspondingsequence (or plurality, or series) of holographic reconstructions, whichare formed by irradiating the SLM with light from a light source. Eachlight pattern in the sequence of light patterns may correspond to adifferent respective hologram. Alternatively, or additionally, two ormore light patterns in a sequence of light patterns may correspond to acommon hologram, combined with a different respective grating functionfor each different light pattern in the sequence. That is, thedifference between two light patterns in a sequence may comprise adifference in the position or location of their respective holographicreconstructions (and their respective holographic replay fields) on aholographic replay plane.

Each light pattern of a sequence of light patterns for projection may beconfigured such that only one detection element of the plurality ofdetection elements should receive light corresponding to the holographicreconstruction of that light pattern, at a time. In some cases, thedetection element that should receive light may change with eachsuccessive light pattern of the sequence of light patterns. In somecases, more than detection element may be expected to receive light at atime, but the specific combination of elements that are expected toreceive light may change with each light pattern in the sequence oflight patterns.

The detection elements may be located substantially at the holographicreplay plane, at which an (intermediate) holographic reconstruction ofthe light pattern is formed. Alternatively, or additionally, some or allof the detection elements may be located at an image plane, at which animage of an intermediate holographic reconstruction is formed.Alternatively, or additionally, some or all of the detection elementsmay be located

upstream of the holographic replay plane (i.e. between the SLM and theholographic replay plane) and/or downstream of the holographic replayplane (e.g. between the holographic replay plane and an image plane).Therefore, in some cases, the holographic reconstruction may not yet befully formed, and/or may not be properly focused, at the location of oneor more of the detection elements. For example, in some cases theholographic replay field may be located at the retina of the observer'seye, using the eye's lens as a Fourier lens to form the holographicreconstruction. Therefore, any monitoring of the light from the SLMwould be upstream of the Fourier lens, in such an arrangement.

The plurality of positions, of the holographic reconstruction, for whichthere is a respective plurality of light detection elements, may be aplurality of positions of a part or parts of the holographicreconstruction itself. For example, they may comprise a plurality ofpositions that a light spot (or light spots) within a holographicreconstruction may move to, when a grating or other function is used totranslate the holographic reconstruction on its holographic replayplane.

For example, they may comprise a plurality of positions at which a lightspot may (or may not) occur, within a holographic reconstruction,dependent on which one of a selection of possible holograms isirradiated, at a given time.

The plurality of light detection elements may be arranged to detectlight ‘corresponding to’ a respective plurality of positions of theholographic reconstruction in the sense that the light detectionelements may not actually be located at the holographic reply plane, atwhich the holographic reconstruction is fully formed and focused.Instead, they may be at another plane, which may be parallel to theholographic reply plane. In other words, they may be located at adifferent point in the trajectory or lightpath of the light, other thanat the holographic replay plane. The light detection elements may detectlight that is travelling towards a holographic replay plane, to form aholographic reconstruction, and/or light that is being projected onwardsfrom its holographic replay plane, having already formed a holographicreconstruction.

The light detection elements may be located at positions at which thelight that may form (or may have formed) one or more light spots withina holographic reconstruction would be expected to be present, at leastat certain times or under certain circumstances. Therefore, the lightdetection elements may be located on a plane that is upstream of theholographic replay plane for a particular holographic reconstruction, ata respective plurality of positions at which light may be detected,wherein that light is on a trajectory (or lightpath) towards theholographic replay plane, where it may form one or more light spotswithin the holographic reconstruction. Alternatively, or additionally,the light detection elements may be located on a plane that isdownstream of the holographic replay plane for a particular holographicreconstruction, at a respective plurality of positions at which lightmay be detected, wherein that light is on a trajectory (or lightpath)away from the holographic replay plane, and wherein that light may havepreviously formed one or more light spots within the holographicreconstruction.

The light pattern for projection may comprise a primary light patternregion and a secondary light pattern region. Each position of theplurality of positions, at which the light detection elements arearranged to detect light, may be within the secondary light patternregion.

The secondary light pattern region may be different to the primary lightpattern region, optionally, for example, the second light pattern regionmay be spatially-separated from the primary light pattern region. Forexample, the primary light pattern region may comprise image contentthat is intended to be viewed by a viewer. Conversely, the secondarylight pattern region may not comprise image content that is intended tobe viewed by a viewer. For example, the hologram may be a hologram of atarget image, wherein the primary light pattern corresponds to the imagecontent of the target image and the secondary light pattern correspondsto additional hologram content. For example, the hologram may be ahologram of a modified target image, which has a marker or identifieradded to the original image content. The primary light pattern regionmay correspond to the original image content of the target image and thesecondary light pattern may correspond to the added marker oridentifier. The added marker or identifier may be referred to as being a“holographic fingerprint”.

A block, or baffle, or barrier may be provided, to prevent light withinthe secondary light pattern region from being transmitted substantiallybeyond the holographic replay plane. An aperture or opening or windowmay be provided to allow light within the primary light pattern regionto be transmitted substantially beyond the holographic replay plane.

Both the secondary light pattern region and the primary light patternregion may be comprised within a common order of holographic replayfield, on the holographic replay plane. For example, they may both becomprised within the zeroth-order holographic replay field.

Alternatively, the secondary light pattern region and the primary lightpattern region may be comprised within different respective orders ofholographic replay field, on the holographic replay plane. For example,the primary light pattern region may be comprised within thezeroth-order holographic replay field and the secondary light patternregion may be comprised within a first-order holographic replay field.

The locations of the plurality of positions that are respectivelymonitored by the plurality of detection elements may coincide with thesecondary light pattern region. If the detection elements are providedupstream of the holographic replay plane, the plurality of positionsthat are respectively monitored by the plurality of detection elementsmay coincide with the light that will form the secondary light patternregion when it reaches the holographic replay plane. If the detectionelements are provided downstream of the holographic replay plane, theplurality of positions that are respectively monitored by the pluralityof detection elements may coincide with the light that previously formedthe secondary light pattern region, on the holographic replay plane. Thelocations of the plurality of positions that are respectively monitoredby the plurality of detection elements may therefore be within thezeroth-order holographic replay field or may be within a higher-orderrepeat of a zeroth-order holographic replay field. The plurality ofpositions within a higher-order repeat may be substantially adjacent tothe zeroth-order holographic replay field.

If the plurality of positions that are respectively monitored by theplurality of detection elements are within a higher-order repeat of azeroth-order holographic replay field, the detector array may besubstantially coplanar with, or substantially perpendicular to, theholographic replay plane.

The light pattern may comprise an array of light spots for lightdetection and ranging, “LIDAR”. The holographic projector may becomprised within a LIDAR system, for observing or interrogating a sceneor target.

A LIDAR controller may be provided and arranged to move or change aholographic replay field, in which a holographic reconstruction isformed, in time, such that each light spot of the array of light spotseffectively occupies a plurality of different positions on theholographic replay plane during a scan period. The array of light spotsmoves as a whole, when the holographic replay field is moved or changed.The movement of the light spots to their different positions during thescan period may correlate with the plurality of positions respectivelymonitored by the plurality of detection elements. In other words, theplurality of detection elements may be located to capture a light signalfrom one or more specific light spots, within the array of light spots,as it/they move(s) around.

For example, a light spot (or spots or other light form) that does notcontribute to the ‘main’ array of light spots that is used for lightdetection and ranging (LIDAR), but is nonetheless comprised within thesame holographic reconstruction that includes the ‘main’ array of lightspots, may move between two or more of the monitored plurality ofdifferent positions on the holographic replay plane, during a scanperiod. That light spot (or spots or other light form) may comprise aholographic identifier or ‘fingerprint’ for a holographicreconstruction. One or more characteristic(s) of the fingerprint may bemonitored, for determination of whether the holographic projector isoperating correctly, at a given time. One or more signals that aregenerated due to the detection of the fingerprint, or part of thefingerprint, may be monitored and may be compared to one or morecorresponding expected signals, for determination of whether theholographic projector is operating correctly and safely, at a giventime.

The detection of light by at least one detection element of theplurality of detection elements may be used to trigger the start of atime-of-flight measurement using a light spot of the array of lightspots.

The light pattern may be an image for a head-up display.

According to an aspect, a method is provided of monitoring operation ofa holographic projector, the holographic projector comprising: a spatiallight modulator arranged to display a hologram of a light pattern and tospatially-modulate light to form a holographic reconstruction, whereinthe holographic reconstruction is spatially-separated from the spatiallight modulator; a detector array comprising a plurality of lightdetection elements arranged to detect light at a respective plurality ofpositions of the holographic reconstruction and to provide a respectiveplurality of output signals related to light detection; and a faultdetection circuit. The method comprises displaying, at the spatial lightmodulator, a hologram of a light pattern; illuminating the spatial lightmodulator, to form a holographic reconstruction of the light pattern;detecting, at the detector array, a light signal corresponding to theholographic reconstruction; receiving, at the fault detection circuit,an output signal from a light detection element, within the detectorarray, relating to the detected light signal corresponding to theholographic reconstruction; and comparing the received output signalwith one or more of a plurality of expected signals, which are based onthe light distribution of the light pattern.

The one or more of a plurality of expected signals may be time-varying.The light pattern for projection may comprise a primary light patternregion and a secondary light pattern region, wherein each position ofthe plurality of positions may be within the secondary light patternregion.

If the holographic projection is operating properly and safely, theformed holographic reconstruction should correspond to the lightpattern. The fault detection circuit may be arranged to have storedthereon, or to have access to, the plurality of expected signals, whichwould be expected to be received from the detector array, if theholographic projector was operating correctly, and properly forming theholographic reconstruction(s) of one or more particular light patterns,which are represented by one or more corresponding holograms, displayedon the spatial light modulator (SLM). The fault detection circuit may bearranged to access one or more particular expected signals, whichcorrespond to the light pattern for a currently-displayed hologram, andto compare the received output signal from the light detection elementto that one or more particular expected signals.

The method may further comprise the fault detection further determining,as a result of said comparison, whether any difference exists betweenthe received output signal and the one or more of a plurality ofexpected signals, and optionally also determining whether thatdifference is greater than an acceptability value.

The method may further comprise controlling the holographic projector sothat, if it is determined that a difference exists between the receivedoutput signal and the one or more of a plurality of expected signals,and/or if it is determined that a difference exists that is greater thanan acceptability value, further light projection is altered orprevented. The alteration or prevention of the further light projectionmay comprise taking steps to stop or pause illumination of the SLM by alight source, or to change a parameter of the illumination, and/or itmay comprise activating one or more barriers or shutters, along thelight path, within the holographic projector, between the light sourceand the observer.

The holographic projector according to any of the above aspects may becomprised within a light detection and ranging, “LIDAR”, system.

The method according to any of the above aspects may be acomputer-implemented method.

A computer program may be provided comprising instructions which, whenexecuted by data processing apparatus, causes the data processingapparatus to perform a method according to any of the above aspects.

A computer readable medium may be provided, storing a computer programaccording to the above aspect.

The term “hologram” is used to refer to the recording which containsamplitude information or phase information, or some combination thereof,regarding the object. The term “holographic reconstruction” is used torefer to the optical reconstruction of the object which is formed byilluminating the hologram. The system disclosed herein is described as a“holographic projector” because the holographic reconstruction is a realimage and spatially-separated from the hologram. The term “replay field”is used to refer to the 2D area within which the holographicreconstruction is formed and fully focused. If the hologram is displayedon a spatial light modulator comprising pixels, the replay field will berepeated in the form of a plurality diffracted orders wherein eachdiffracted order is a replica of the zeroth-order replay field. Thezeroth-order replay field generally corresponds to the preferred orprimary replay field because it is the brightest replay field. Unlessexplicitly stated otherwise, the term “replay field” should be taken asreferring to the zeroth-order replay field. The term “replay plane” isused to refer to the plane in space containing all the replay fields.The terms “image”, “replay image” and “image region” refer to areas ofthe replay field illuminated by light of the holographic reconstruction.In some embodiments, the “image” may comprise discrete spots which maybe referred to as “image spots” or, for convenience only, “imagepixels”.

The terms “encoding”, “writing” or “addressing” are used to describe theprocess of providing the plurality of pixels of the SLM with arespective plurality of control values which respectively determine themodulation level of each pixel. It may be said that the pixels of theSLM are configured to “display” a light modulation distribution inresponse to receiving the plurality of control values. Thus, the SLM maybe said to “display” a hologram and the hologram may be considered anarray of light modulation values or levels.

It has been found that a holographic reconstruction of acceptablequality can be formed from a “hologram” containing only phaseinformation related to the Fourier transform of the original object.Such a holographic recording may be referred to as a phase-onlyhologram. Embodiments relate to a phase-only hologram but the presentdisclosure is equally applicable to amplitude-only holography.

The present disclosure is also equally applicable to forming aholographic reconstruction using amplitude and phase information relatedto the Fourier transform of the original object. In some embodiments,this is achieved by complex modulation using a so-called fully complexhologram which contains both amplitude and phase information related tothe original object. Such a hologram may be referred to as afully-complex hologram because the value (grey level) assigned to eachpixel of the hologram has an amplitude and phase component. The value(grey level) assigned to each pixel may be represented as a complexnumber having both amplitude and phase components. In some embodiments,a fully-complex computer-generated hologram is calculated.

Reference may be made to the phase value, phase component, phaseinformation or, simply, phase of pixels of the computer-generatedhologram or the spatial light modulator as shorthand for “phase-delay”.That is, any phase value described is, in fact, a number (e.g. in therange 0 to 2π) which represents the amount of phase retardation providedby that pixel. For example, a pixel of the spatial light modulatordescribed as having a phase value of π/2 will retard the phase ofreceived light by π/2 radians. In some embodiments, each pixel of thespatial light modulator is operable in one of a plurality of possiblemodulation values (e.g. phase delay values). The term “grey level” maybe used to refer to the plurality of available modulation levels. Forexample, the term “grey level” may be used for convenience to refer tothe plurality of available phase levels in a phase-only modulator eventhough different phase levels do not provide different shades of grey.The term “grey level” may also be used for convenience to refer to theplurality of available complex modulation levels in a complex modulator.

The hologram therefore comprises an array of grey levels—that is, anarray of light modulation values such as an array of phase-delay valuesor complex modulation values. The hologram is also considered adiffractive pattern because it is a pattern that causes diffraction whendisplayed on a spatial light modulator and illuminated with light havinga wavelength comparable to, generally less than, the pixel pitch of thespatial light modulator. Reference is made herein to combining thehologram with other diffractive patterns such as diffractive patternsfunctioning as a lens or grating. For example, a diffractive patternfunctioning as a grating may be combined with a hologram to translatethe replay field on the replay plane or a diffractive patternfunctioning as a lens may be combined with a hologram to focus theholographic reconstruction on a replay plane in the near field.

Although different examples, arrangements, aspects, embodiments andgroups of embodiments may be disclosed separately in the detaileddescription which follows, any feature of any examples, arrangement,aspect, embodiment or group of embodiments may be combined with anyother feature or combination of features of any embodiment or group ofembodiments. That is, all possible combinations and permutations offeatures disclosed in the present disclosure are envisaged.

BRIEF DESCRIPTION OF THE FIGURES

The above, as well as additional, features will be better understoodthrough the following illustrative and non-limiting detailed descriptionof example embodiments, with reference to the appended drawings.

FIG. 1 is a schematic showing a reflective SLM producing a holographicreconstruction on a screen;

FIG. 2A illustrates a first iteration of an example Gerchberg-Saxtontype algorithm;

FIG. 2B illustrates the second and subsequent iterations of the exampleGerchberg-Saxton type algorithm;

FIG. 2C illustrates alternative second and subsequent iterations of theexample Gerchberg-Saxton type algorithm;

FIG. 3 is a schematic of a reflective LCOS SLM;

FIG. 4 is a schematic of a light source system, or holographicprojector, that may be employed as part of a holographic Light Detectionand Ranging (LIDAR) system;

FIG. 5 is a schematic of a light detector system that may be employed aspart of a holographic Light Detection and Ranging (LIDAR) system;

FIG. 6 is a schematic of a combined light source and detector systemthat may be employed as part of a holographic Light Detection andRanging (LIDAR) system;

FIGS. 7A and 7B show an improved holographic Light Detection and Ranging(LIDAR) system, in accordance with embodiments;

FIG. 8A shows a holographic reconstruction at a first grating position;

FIG. 8B shows the holographic reconstruction of FIG. 8A at fourthgrating position;

FIG. 8C shows the holographic reconstruction of FIG. 8A at thirteenthgrating position;

FIG. 8D shows the holographic reconstruction of FIG. 8A at sixteenthgrating position;

FIG. 8E shows expected signals from four respective light detectors,configured to detect the holographic reconstruction of FIG. 8A at aplurality of grating positions;

FIG. 9 shows a schematic representation of a safeguarding method for animproved LIDAR system, in accordance with embodiments;

FIG. 10A shows a first arrangement for an improved LIDAR system, inaccordance with further embodiments;

FIG. 10B shows a second arrangement for an improved LIDAR system, inaccordance with further embodiments;

FIG. 10C shows a third arrangement for an improved LIDAR system, inaccordance with further embodiments; and

FIG. 11 shows a waveguide pupil expander comprising a pair of parallelmirrors in accordance with some embodiments;

FIG. 12 shows a waveguide pupil expander comprising an optical slab oftransparent material in accordance with other embodiments;

FIG. 13 shows an indirect view holographic system including anintermediate holographic replay screen and waveguide pupil expander; and

FIG. 14 shows a direct view holographic system including a waveguidepupil expander.

All the figures are schematic, not necessarily to scale, and generallyonly show parts which are necessary to elucidate example embodiments,wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter withreference to the accompanying drawings. That which is encompassed by theclaims may, however, be embodied in many different forms and should notbe construed as limited to the embodiments set forth herein; rather,these embodiments are provided by way of example. Furthermore, likenumbers refer to the same or similar elements or components throughout.

The present invention is not restricted to the embodiments described inthe following but extends to the full scope of the appended claims. Thatis, the present invention may be embodied in different forms and shouldnot be construed as limited to the described embodiments, which are setout for the purpose of illustration.

Terms of a singular form may include plural forms unless specifiedotherwise.

A structure described as being formed at an upper portion/lower portionof another structure or on/under the other structure should be construedas including a case where the structures contact each other and,moreover, a case where a third structure is disposed there between.

In describing a time relationship—for example, when the temporal orderof events is described as “after”, “subsequent”, “next”, “before” orsuchlike—the present disclosure should be taken to include continuousand non-continuous events unless otherwise specified. For example, thedescription should be taken to include a case which is not continuousunless wording such as “just”, “immediate” or “direct” is used.

Although the terms “first”, “second”, etc. may be used herein todescribe various elements, these elements are not to be limited by theseterms. These terms are only used to distinguish one element fromanother. For example, a first element could be termed a second element,and, similarly, a second element could be termed a first element,without departing from the scope of the appended claims.

Features of different embodiments may be partially or overall coupled toor combined with each other, and may be variously inter-operated witheach other. Some embodiments may be carried out independently from eachother, or may be carried out together in co-dependent relationship.

Optical Configuration

FIG. 1 shows an embodiment in which a computer-generated hologram isencoded on a single spatial light modulator. The computer-generatedhologram is a Fourier transform of the object for reconstruction. It maytherefore be said that the hologram is a Fourier domain or frequencydomain or spectral domain representation of the object. In thisembodiment, the spatial light modulator is a reflective liquid crystalon silicon, “LCOS”, device. The hologram is encoded on the spatial lightmodulator and a holographic reconstruction is formed at a replay field,for example, a light receiving surface such as a screen or diffuser.

A light source 110, for example a laser or laser diode, is disposed toilluminate the SLM 140 via a collimating lens 111. The collimating lenscauses a generally planar wavefront of light to be incident on the SLM.In FIG. 1, the direction of the wavefront is off-normal (e.g. two orthree degrees away from being truly orthogonal to the plane of thetransparent layer). However, in other embodiments, the generally planarwavefront is provided at normal incidence and a beam splitterarrangement is used to separate the input and output optical paths. Inthe embodiment shown in FIG. 1, the arrangement is such that light fromthe light source is reflected off a mirrored rear surface of the SLM andinteracts with a light-modulating layer to form an exit wavefront 112.The exit wavefront 112 is applied to optics including a Fouriertransform lens 120, having its focus at a screen 125. More specifically,the Fourier transform lens 120 receives a beam of modulated light fromthe SLM 140 and performs a frequency-space transformation to produce aholographic reconstruction at the screen 125.

Notably, in this type of holography, each pixel of the hologramcontributes to the whole reconstruction. There is not a one-to-onecorrelation between specific points (or image pixels) on the replayfield and specific light-modulating elements (or hologram pixels). Inother words, modulated light exiting the light-modulating layer isdistributed across the replay field.

In these embodiments, the position of the holographic reconstruction inspace is determined by the dioptric (focusing) power of the Fouriertransform lens. In the embodiment shown in FIG. 1, the Fourier transformlens is a physical lens. That is, the Fourier transform lens is anoptical Fourier transform lens and the Fourier transform is performedoptically. Any lens can act as a Fourier transform lens but theperformance of the lens will limit the accuracy of the Fourier transformit performs. The skilled person understands how to use a lens to performan optical Fourier transform.

Hologram Calculation

In some embodiments, the computer-generated hologram is a Fouriertransform hologram, or simply a Fourier hologram or Fourier-basedhologram, in which an image is reconstructed in the far field byutilising the Fourier transforming properties of a positive lens. TheFourier hologram is calculated by Fourier transforming the desired lightfield in the replay plane back to the lens plane. Computer-generatedFourier holograms may be calculated using Fourier transforms.

A Fourier transform hologram may be calculated using an algorithm suchas the Gerchberg-Saxton algorithm. Furthermore, the Gerchberg-Saxtonalgorithm may be used to calculate a hologram in the Fourier domain(i.e. a Fourier transform hologram) from amplitude-only information inthe spatial domain (such as a photograph). The phase information relatedto the object is effectively “retrieved” from the amplitude-onlyinformation in the spatial domain. In some embodiments, acomputer-generated hologram is calculated from amplitude-onlyinformation using the Gerchberg-Saxton algorithm or a variation thereof.

The Gerchberg Saxton algorithm considers the situation when intensitycross-sections of a light beam, IA(x, y) and IB(x, y), in the planes Aand B respectively, are known and IA(x, y) and IB(x, y) are related by asingle Fourier transform. With the given intensity cross-sections, anapproximation to the phase distribution in the planes A and B, ΨA(x, y)and ΨB(x, y) respectively, is found. The Gerchberg-Saxton algorithmfinds solutions to this problem by following an iterative process. Morespecifically, the Gerchberg-Saxton algorithm iteratively applies spatialand spectral constraints while repeatedly transferring a data set(amplitude and phase), representative of IA(x, y) and IB(x, y), betweenthe spatial domain and the Fourier (spectral or frequency) domain. Thecorresponding computer-generated hologram in the spectral domain isobtained through at least one iteration of the algorithm. The algorithmis convergent and arranged to produce a hologram representing an inputimage. The hologram may be an amplitude-only hologram, a phase-onlyhologram or a fully complex hologram.

In some embodiments, a phase-only hologram is calculated using analgorithm based on the Gerchberg-Saxton algorithm such as described inBritish patent 2,498,170 or 2,501,112 which are hereby incorporated intheir entirety by reference. However, embodiments disclosed hereindescribe calculating a phase-only hologram by way of example only. Inthese embodiments, the Gerchberg-Saxton algorithm retrieves the phaseinformation Ψ[u, v] of the Fourier transform of the data set which givesrise to a known amplitude information T[x, y], wherein the amplitudeinformation T[x, y] is representative of a target image (e.g. aphotograph). Since the magnitude and phase are intrinsically combined inthe Fourier transform, the transformed magnitude and phase containuseful information about the accuracy of the calculated data set. Thus,the algorithm may be used iteratively with feedback on both theamplitude and the phase information. However, in these embodiments, onlythe phase informationΨ[u, v] is used as the hologram to form aholographic representative of the target image at an image plane. Thehologram is a data set (e.g. 2D array) of phase values.

In other embodiments, an algorithm based on the Gerchberg-Saxtonalgorithm is used to calculate a fully-complex hologram. A fully-complexhologram is a hologram having a magnitude component and a phasecomponent. The hologram is a data set (e.g. 2D array) comprising anarray of complex data values wherein each complex data value comprises amagnitude component and a phase component.

In some embodiments, the algorithm processes complex data and theFourier transforms are complex Fourier transforms. Complex data may beconsidered as comprising (i) a real component and an imaginary componentor (ii) a magnitude component and a phase component. In someembodiments, the two components of the complex data are processeddifferently at various stages of the algorithm.

FIG. 2A illustrates the first iteration of an algorithm in accordancewith some embodiments for calculating a phase-only hologram. The inputto the algorithm is an input image 210 comprising a 2D array of pixelsor data values, wherein each pixel or data value is a magnitude, oramplitude, value. That is, each pixel or data value of the input image210 does not have a phase component. The input image 210 may thereforebe considered a magnitude-only or amplitude-only or intensity-onlydistribution. An example of such an input image 210 is a photograph orone frame of video comprising a temporal sequence of frames. The firstiteration of the algorithm starts with a data forming step 202Acomprising assigning a random phase value to each pixel of the inputimage, using a random phase distribution (or random phase seed) 230, toform a starting complex data set wherein each data element of the setcomprising magnitude and phase. It may be said that the starting complexdata set is representative of the input image in the spatial domain.

First processing block 250 receives the starting complex data set andperforms a complex Fourier transform to form a Fourier transformedcomplex data set. Second processing block 253 receives the Fouriertransformed complex data set and outputs a hologram 280A. In someembodiments, the hologram 280A is a phase-only hologram. In theseembodiments, second processing block 253 quantises each phase value andsets each amplitude value to unity in order to form hologram 280A. Eachphase value is quantised in accordance with the phase-levels which maybe represented on the pixels of the spatial light modulator which willbe used to “display” the phase-only hologram. For example, if each pixelof the spatial light modulator provides 256 different phase levels, eachphase value of the hologram is quantised into one phase level of the 256possible phase levels. Hologram 280A is a phase-only Fourier hologramwhich is representative of an input image. In other embodiments, thehologram 280A is a fully complex hologram comprising an array of complexdata values (each including an amplitude component and a phasecomponent) derived from the received Fourier transformed complex dataset. In some embodiments, second processing block 253 constrains eachcomplex data value to one of a plurality of allowable complex modulationlevels to form hologram 280A. The step of constraining may includesetting each complex data value to the nearest allowable complexmodulation level in the complex plane. It may be said that hologram 280Ais representative of the input image in the spectral or Fourier orfrequency domain. In some embodiments, the algorithm stops at thispoint.

However, in other embodiments, the algorithm continues as represented bythe dotted arrow in FIG. 2A. In other words, the steps which follow thedotted arrow in FIG. 2A are optional (i.e. not essential to allembodiments).

Third processing block 256 receives the modified complex data set fromthe second processing block 253 and performs an inverse Fouriertransform to form an inverse Fourier transformed complex data set. Itmay be said that the inverse Fourier transformed complex data set isrepresentative of the input image in the spatial domain.

Fourth processing block 259 receives the inverse Fourier transformedcomplex data set and extracts the distribution of magnitude values 211Aand the distribution of phase values 213A. Optionally, the fourthprocessing block 259 assesses the distribution of magnitude values 211A.Specifically, the fourth processing block 259 may compare thedistribution of magnitude values 211A of the inverse Fourier transformedcomplex data set with the input image 510 which is itself, of course, adistribution of magnitude values. If the difference between thedistribution of magnitude values 211A and the input image 210 issufficiently small, the fourth processing block 259 may determine thatthe hologram 280A is acceptable. That is, if the difference between thedistribution of magnitude values 211A and the input image 210 issufficiently small, the fourth processing block 259 may determine thatthe hologram 280A is a sufficiently-accurate representative of the inputimage 210. In some embodiments, the distribution of phase values 213A ofthe inverse Fourier transformed complex data set is ignored for thepurpose of the comparison. It will be appreciated that any number ofdifferent methods for comparing the distribution of magnitude values211A and the input image 210 may be employed and the present disclosureis not limited to any particular method. In some embodiments, a meansquare difference is calculated and if the mean square difference isless than a threshold value, the hologram 280A is deemed acceptable. Ifthe fourth processing block 259 determines that the hologram 280A is notacceptable, a further iteration of the algorithm may be performed.However, this comparison step is not essential and in other embodiments,the number of iterations of the algorithm performed is predetermined orpreset or user-defined.

FIG. 2B represents a second iteration of the algorithm and any furtheriterations of the algorithm. The distribution of phase values 213A ofthe preceding iteration is fed-back through the processing blocks of thealgorithm. The distribution of magnitude values 211A is rejected infavour of the distribution of magnitude values of the input image 210.In the first iteration, the data forming step 202A formed the firstcomplex data set by combining distribution of magnitude values of theinput image 210 with a random phase distribution 230. However, in thesecond and subsequent iterations, the data forming step 202B comprisesforming a complex data set by combining (i) the distribution of phasevalues 213A from the previous iteration of the algorithm with (ii) thedistribution of magnitude values of the input image 210.

The complex data set formed by the data forming step 202B of FIG. 2B isthen processed in the same way described with reference to FIG. 2A toform second iteration hologram 280B. The explanation of the process isnot therefore repeated here. The algorithm may stop when the seconditeration hologram 280B has been calculated. However, any number offurther iterations of the algorithm may be performed. It will beunderstood that the third processing block 256 is only required if thefourth processing block 259 is required or a further iteration isrequired. The output hologram 280B generally gets better with eachiteration. However, in practice, a point is usually reached at which nomeasurable improvement is observed or the positive benefit of performinga further iteration is out-weighted by the negative effect of additionalprocessing time. Hence, the algorithm is described as iterative andconvergent.

FIG. 2C represents an alternative embodiment of the second andsubsequent iterations. The distribution of phase values 213A of thepreceding iteration is fed-back through the processing blocks of thealgorithm. The distribution of magnitude values 211A is rejected infavour of an alternative distribution of magnitude values. In thisalternative embodiment, the alternative distribution of magnitude valuesis derived from the distribution of magnitude values 211 of the previousiteration. Specifically, processing block 258 subtracts the distributionof magnitude values of the input image 210 from the distribution ofmagnitude values 211 of the previous iteration, scales that differenceby a gain factor a and subtracts the scaled difference from the inputimage 210. This is expressed mathematically by the following equations,wherein the subscript text and numbers indicate the iteration number:

R _(n+1)[x, y]=F′{exp(iψ _(n)[u, v])}

ψ_(n)[u, v]=∠F{η·exp(i∠R _(n)[x, y])}

η=T[x, y]−α(|R _(n)[x, y]|−T[x, y])

-   -   where:    -   F′ is the inverse Fourier transform;    -   F is the forward Fourier transform;    -   R[x, y] is the complex data set output by the third processing        block 256;    -   T[x, y] is the input or target image;    -   ∠ is the phase component;    -   Ψ is the phase-only hologram 280B;    -   η is the new distribution of magnitude values 211B; and    -   α is the gain factor.

The gain factor α may be fixed or variable. In some embodiments, thegain factor α is determined based on the size and rate of the incomingtarget image data. In some embodiments, the gain factor α is dependenton the iteration number. In some embodiments, the gain factor a issolely function of the iteration number.

The embodiment of FIG. 2C is the same as that of FIG. 2A and FIG. 2B inall other respects. It may be said that the phase-only hologram Ψ(u, v)comprises a phase distribution in the frequency or Fourier domain.

In some embodiments, the Fourier transform is performed using thespatial light modulator. Specifically, the hologram data is combinedwith second data providing optical power. That is, the data written tothe spatial light modulation comprises hologram data representing theobject and lens data representative of a lens. When displayed on aspatial light modulator and illuminated with light, the lens dataemulates a physical lens—that is, it brings light to a focus in the sameway as the corresponding physical optic. The lens data thereforeprovides optical, or focusing, power. In these embodiments, the physicalFourier transform lens 120 of FIG. 1 may be omitted. It is known how tocalculate data representative of a lens. The data representative of alens may be referred to as a software lens. For example, a phase-onlylens may be formed by calculating the phase delay caused by each pointof the lens owing to its refractive index and spatially-variant opticalpath length. For example, the optical path length at the centre of aconvex lens is greater than the optical path length at the edges of thelens. An amplitude-only lens may be formed by a Fresnel zone plate. Itis also known in the art of computer-generated holography how to combinedata representative of a lens with a hologram so that a Fouriertransform of the hologram can be performed without the need for aphysical Fourier lens. In some embodiments, lensing data is combinedwith the hologram by simple addition such as simple vector addition. Insome embodiments, a physical lens is used in conjunction with a softwarelens to perform the Fourier transform. Alternatively, in otherembodiments, the Fourier transform lens is omitted altogether such thatthe holographic reconstruction takes place in the far-field. In furtherembodiments, the hologram may be combined in the same way with gratingdata—that is, data arranged to perform the function of a grating such asimage steering. Again, it is known in the field how to calculate suchdata. For example, a phase-only grating may be formed by modelling thephase delay caused by each point on the surface of a blazed grating. Anamplitude-only grating may be simply superimposed with an amplitude-onlyhologram to provide angular steering of the holographic reconstruction.The second data providing lensing and/or steering may be referred to asa light processing function or light processing pattern to distinguishfrom the hologram data which may be referred to as an image formingfunction or image forming pattern.

In some embodiments, the Fourier transform is performed jointly by aphysical Fourier transform lens and a software lens. That is, someoptical power which contributes to the Fourier transform is provided bya software lens and the rest of the optical power which contributes tothe Fourier transform is provided by a physical optic or optics.

In some embodiments, there is provided a real-time engine arranged toreceive image data and calculate holograms in real-time using thealgorithm. In some embodiments, the image data is a video comprising asequence of image frames. In other embodiments, the holograms arepre-calculated, stored in computer memory and recalled as needed fordisplay on a SLM. That is, in some embodiments, there is provided arepository of predetermined holograms.

Embodiments relate to Fourier holography and Gerchberg-Saxton typealgorithms by way of example only. The present disclosure is equallyapplicable to Fresnel holography and Fresnel holograms which may becalculated by a similar method. The present disclosure is alsoapplicable to holograms calculated by other techniques such as thosebased on point cloud methods.

Light Modulation

A spatial light modulator may be used to display the diffractive patternincluding the computer-generated hologram. If the hologram is aphase-only hologram, a spatial light modulator which modulates phase isrequired. If the hologram is a fully-complex hologram, a spatial lightmodulator which modulates phase and amplitude may be used or a firstspatial light modulator which modulates phase and a second spatial lightmodulator which modulates amplitude may be used.

In some embodiments, the light-modulating elements (i.e. the pixels) ofthe spatial light modulator are cells containing liquid crystal. Thatis, in some embodiments, the spatial light modulator is a liquid crystaldevice in which the optically-active component is the liquid crystal.Each liquid crystal cell is configured to selectively-provide aplurality of light modulation levels. That is, each liquid crystal cellis configured at any one time to operate at one light modulation levelselected from a plurality of possible light modulation levels. Eachliquid crystal cell is dynamically-reconfigurable to a different lightmodulation level from the plurality of light modulation levels. In someembodiments, the spatial light modulator is a reflective liquid crystalon silicon (LCOS) spatial light modulator but the present disclosure isnot restricted to this type of spatial light modulator.

A LCOS device provides a dense array of light modulating elements, orpixels, within a small aperture (e.g. a few centimetres in width). Thepixels are typically approximately 10 microns or less which results in adiffraction angle of a few degrees meaning that the optical system canbe compact. It is easier to adequately illuminate the small aperture ofa LCOS SLM than it is the larger aperture of other liquid crystaldevices. An LCOS device is typically reflective which means that thecircuitry which drives the pixels of a LCOS SLM can be buried under thereflective surface. The results in a higher aperture ratio. In otherwords, the pixels are closely packed meaning there is very little deadspace between the pixels. This is advantageous because it reduces theoptical noise in the replay field. A LCOS SLM uses a silicon backplanewhich has the advantage that the pixels are optically flat. This isparticularly important for a phase modulating device.

A suitable LCOS SLM is described below, by way of example only, withreference to FIG. 3. An LCOS device is formed using a single crystalsilicon substrate 302. It has a 2D array of square planar aluminiumelectrodes 301, spaced apart by a gap 301 a, arranged on the uppersurface of the substrate. Each of the electrodes 301 can be addressedvia circuitry 302 a buried in the substrate 302. Each of the electrodesforms a respective planar mirror. An alignment layer 303 is disposed onthe array of electrodes, and a liquid crystal layer 304 is disposed onthe alignment layer 303. A second alignment layer 305 is disposed on theplanar transparent layer 306, e.g. of glass. A single transparentelectrode 307 e.g. of ITO is disposed between the transparent layer 306and the second alignment layer 305.

Each of the square electrodes 301 defines, together with the overlyingregion of the transparent electrode 307 and the intervening liquidcrystal material, a controllable phase-modulating element 308, oftenreferred to as a pixel. The effective pixel area, or fill factor, is thepercentage of the total pixel which is optically active, taking intoaccount the space between pixels 301 a. By control of the voltageapplied to each electrode 301 with respect to the transparent electrode307, the properties of the liquid crystal material of the respectivephase modulating element may be varied, thereby to provide a variabledelay to light incident thereon. The effect is to provide phase-onlymodulation to the wavefront, i.e. no amplitude effect occurs.

The described LCOS SLM outputs spatially modulated light in reflection.Reflective LCOS SLMs have the advantage that the signal lines, gatelines and transistors are below the mirrored surface, which results inhigh fill factors (typically greater than 90%) and high resolutions.Another advantage of using a reflective LCOS spatial light modulator isthat the liquid crystal layer can be half the thickness than would benecessary if a transmissive device were used. This greatly improves theswitching speed of the liquid crystal (a key advantage for theprojection of moving video images). However, the teachings of thepresent disclosure may equally be implemented using a transmissive LCOSSLM.

Light Detection and Ranging (‘LIDAR’ or LiDAR’) Systems

It has previously been disclosed that holographic components andtechniques, such as those described herein, may be used to form thebasis of a Light Detection and Ranging (LIDAR) system. The skilledperson will be aware that, in general terms, LIDAR describesarrangements and methods in which the distance to a target may bemeasured by illuminating the target with pulsed laser light andmeasuring a parameter associated the light that is reflected from thetarget, using a sensor or detector. For example, the return times of thereflected light can be measured and may be used to form representations,such as three-dimensional (3D) representations, of the scene or a targetwithin the scene.

WO2019/224052 discloses a holographic projector used to illuminate atarget, or scene, or plane, using so-called ‘structured light’, in orderto observe or interrogate that target (or scene or plane) as part of aLIDAR system. For example, the structured light may be characterised byhaving a particular form and/or shape and/or pattern. The pattern of thestructured light arises from the hologram that is displayed by a spatiallight modulator and illuminated by a laser light source, within theholographic projector. A holographic projector may be arranged toilluminate a plurality of different holograms in sequence (i.e. oneafter the other), to dynamically change the structed light pattern thatis formed on the target.

LIDAR may be further understood by reference to FIGS. 4 to 6 herein, andto the following description.

FIG. 4 shows, by way of example only, a light source system or‘holographic projector’ arranged to project a structured light pattern.The light source system comprises a spatial light modulator (SLM) 402comprising an array of pixels that are configured to display a hologram.In FIG. 4, the SLM 402 is shown as having a 5×5 array of pixels (i.e. 25pixels in total) but it will be appreciated that this is merelyillustrative and that the number and arrangement of pixels may vary fromthe particular example shown.

In FIG. 4 there is also a projection lens 404, via which light travelsfrom the SLM 402, towards a replay field 406. The replay field 406 maycomprise, for example, a light receiving surface such as a screen ordiffuser. The replay field 406 may comprise, for example, a real-worldtarget object or scene. The light source system may be arranged tointerrogate’ or observe a real-world target object or scene and/or toobtain one or more measurements in relation to it and/or to providetarget illumination of one or more features within a real-world targetor scene.

The SLM 402 is arranged to received light from a light source (notshown) such as a laser diode, in order to irradiate the hologram. Thehologram in this example is a computer-generated hologram. The SLM 402is arranged to receive, and to be encoded with, a computer-generatedhologram from a suitable controller (not shown). The SLM 402 may bearranged to receive a plurality of different computer-generatedholograms, and to store, or otherwise have access to, those hologramsfor display at different respective times.

Although not shown in FIG. 4, a Fourier Transform (FT) lens may beincluded in the light source system, to form a holographicreconstruction of the irradiated hologram, at a holographic replayfield. The ‘holographic replay field’ is a two-dimensional area (withina ‘holographic replay plane’) within which a holographic reconstructionof a desired light pattern is formed, upon irradiation of acorresponding hologram. Such an FT lens could be a physical lens (notshown in FIG. 4), or a software lens formed on the SLM or a combinationof a software lens and a physical lens. The skilled reader will befamiliar with FT lenses and their role in forming holographicreconstructions of holograms, and therefore no further description ofthis feature is provided herein.

In some arrangements, the holographic reconstruction of the irradiatedhologram may be formed downstream of SLM 402, such as in free space atan intermediate holographic replay field 408 that is located between theSLM 402 and the projection lens 404. In such embodiments, the projectionlens 404 forms an image of the intermediate holographic replay field408, and projects it onto the holographic replay field 406 that is usedfor light detection and ranging. It will therefore be understood thatholographic replay field 406 is actually an image of the intermediateholographic replay field 408 formed by projection lens 402. In someembodiments, the image of the holographic replay field 406 formed byprojection lens 402 may be a magnified image of the intermediateholographic replay field 408. In these arrangements, the term “replayfield” is generally used to refer to the image of the intermediateholographic replay field 408 because that is the replay field that isactually used.

In some other arrangements, the projection lens 404 contributes to the(optical) Fourier transform of the hologram displayed on the SLM 402,and therefore the holographic replay field is located downstream of boththe SLM 402 and the projection lens 404.

In the system of FIG. 4, the replay field 406 comprises a plurality ofdiscrete light areas—in this example, there are four discrete lightareas, but it will be appreciated that the number of discrete lightareas may vary from the example shown.

Irradiation of the hologram in FIG. 4 results in a holographicreconstruction (and an image of that holographic reconstruction) that,in this example, comprises four discrete light spots; one in each of thefour discrete light areas of the replay field 406. This light pattern isconsidered to be so-called ‘structured light’ because it comprises aplurality of discrete light features or sub-footprints that providelight in a corresponding plurality of discrete areas (or sub-areas) ofthe replay field 406. It will be appreciated that any pattern ofdiscrete light features, such as light spots, separate by areas ofdarkness, may be formed and the brightness of each light feature or spotmay be individually tuned, based on the selection and irradiation of anappropriate respective hologram, on the SLM 402, at any given time.

In FIG. 4, the four discrete light spots are shown as being in fixedpositions, within their respective discrete light areas of the replayfield 406. In some arrangements, light spots (or other light formations)will not remain fixed in position but may be moved or scanned, aroundrespective areas, for example using a software grating. This isdiscussed in more detail in relation to subsequent figures, later inthis description.

In the example shown in FIG. 4, there is not a one-to-one correlationbetween the pixels of the hologram and the light features (or discretelight areas) of the replay field 406. Instead, all the hologram pixelscontribute to all areas of the replay field 406. The structured lightpattern within the replay field may be used to interrogate the scene,for example to help form an accurate image or model (or series of imagesor models) of the scene and/or to make measurements of the scene.

The scene that the light source system (or holographic projector) isarranged to direct structured light onto may not be planar but may havea depth. The holographic projector may therefore be arranged todynamically adjust its operating parameters in order to vary the preciselocation of the replay field 406 and holographic replay plane, toexplore different respective depths within the scene. A lensing functionmay be added to the hologram 402, in order to maintain focus of thestructured light pattern on the plane of interest, at any given time. Insome cases, the holographic replay field may, itself, not be planar, andthus may comprise light spots that are arranged to come into focus atdifferent respective depths within the same 3D reconstruction (orimage). The light source system (or holographic projector) may comprisea lens or other suitable optics to ensure that it has some inherentdepth of field or depth of focus, with respect to any plane of interestthat is being explored, at a given time.

The distance between the SLM (and the hologram) of a holographicprojector and the plane of interest may be referred to as the ‘range’.The range may be measured along a (virtual) line that joins the centreof the SLM (and of the hologram) to the centre of the zeroth-orderreplay field. This line may be referred to as a ‘projection axis.’Therefore, it may be said that the holographic projector of FIG. 4 maybe controlled (for example, using a lensing function or a plurality oflensing functions) so as to vary the range along its projection axis, toenable observation of multiple planes, and thus multiple depths, with atarget or scene. This is discussed further below, in relation tosubsequent figures.

FIG. 5 shows a light detector system that comprises a light detector 502and an imaging lens 504. The light detector 502 comprises a plurality ofindividual light detection elements arranged in an array. There are fourlight detection elements in the example shown in FIG. 5, wherein thoselight detection elements are respectively numbered 1 to 4. The skilledperson will appreciate that this number of light detection elements ismerely an example, and that other sizes and arrangements of array andother numbers of light detection elements are contemplated.

The light detector 502 may comprise, for example, a charge-coupleddevice (CCD) camera, comprising an array of CCD elements. Alternatively,the light detector 502 may be a single-photon avalanche diode (SPAD)array comprising an array of SPAD elements.

The light detector 502 is arranged to receive reflected light from atarget or scene 506 that is to be interrogated or observed. The lightfrom the observed scene 506 travels, via the imaging lens 504, towardsthe light detector 502.

The light detector 502 may comprise, or may be communicatively coupledto, a suitable controller. The controller may be configured to processlight detection signals from the light detector 502 in order to obtainmeasurements of, or make other determinations relating to, the observedscene 506, as will be discussed further below in relation to subsequentfigures.

In the arrangement of FIG. 5, the observed scene 506 comprises objectslabelled A, B, C and D. Each individual light detection element (1, 2,3, 4) of the light detector 502 in FIG. 5 is arranged to receive lightfrom a single respective corresponding object (A, B, C, D) in theobserved scene 506. Each light detection element in the example of FIG.5 is arranged only to receive light from its corresponding object andthus not to receive light from any of the ‘other’ objects within theobserved scene 506. That is; the optics of the light detector system arearranged so that, for example, element 1 receives light from object Aonly, element 2 receives light from object B only, element 3 receiveslight from object C only and element 4 receives light from object Donly. It may therefore be said that there is a one-to-one correlationbetween an individual light detection element (1, 2, 3, 4) and itscorresponding object (A, B, C, D) within the observed scene 506,although the light detection elements and the objects may have differentrespective sizes. Alternatively, element 4 may receive light from objectA only, element 3 may receive light from object B only, element 2 mayreceive light from object C only and element 1 may receive light fromobject D only

Although A, B, C and D are represented in FIG. 5 as being particulargeometric formations (cuboids), it should be appreciated that this isillustrative only, and should not be regarded as limiting. In practice,the individual light detection elements (1, 2, 3, 4) of the lightdetector 502 may be configured to have one-to-one correlation withrespective regions or areas (or sub-regions or sub-areas) of a scene ortarget, from which light is reflected towards the light detector 502.The light detection elements may therefore be configured to receivereflected light from whatever object(s) or formation(s) or being(s)is/are present at the corresponding regions of the scene, at a giventime. For succinctness in the present disclosure, whatever is present ina particular region of a scene, at a given time, is referred to as beingan “object”. The skilled person will understand that each “object” mayactually be a different area of the same object—e.g., car or tree.

In accordance with this disclosure, it is also said that each lightdetection element (1, 2, 3, 4) has a respective or correspondingindividual field of view of the scene (A, B, C, D respectively). Forexample, the individual field of view of light detection element number1 of FIG. 5 enables it to detect reflected light from an object withinindividual field of view A but does not enable it to detect reflectedlight from an object within individual field of view B, C or D. In someembodiments, the individual fields of view (e.g. A, B, C, D) arenon-overlapping. In embodiments, the individual fields of viewcollectively provide substantially continuous coverage of an area of thescene. However, the person skilled in the art will appreciate that, inpractice, there may be “dead zones” between the individual fields ofview within which light cannot be detected by the system. The skilledperson will understand how the imaging lens 504 plays the role ofestablishing the one-to-one correlation between each light detectionelement (1, 2, 3, 4) and individual field of view (A, B, C, D). It isknown in the field of imaging how to design the imaging lens and anynumber of different imaging lens may be used depending on operatingparameters of the system. The skilled person will also appreciate thatthe imaging lens 504 provides a depth of field such that light detectionand ranging of each “object” (A, B, C, D) of FIG. 5 may be conducted atthe same time. In other words, the “range” referred to in thisdisclosure includes a tolerance—for example, the range may be 100+/−2metres, wherein the +/−2 metres reflects the depth of field. Thisexample is illustrative and should not be regarded as limiting—othersizes of range and other depths of filed are contemplated. For theavoidance of doubt, continual adjustment of e.g. the software lensfunction is not required in order to perform light detection and rangingin relation to the four “objects” (A, B, C, D) shown in FIG. 5 becauseof the depth of field provided by imaging lens 504.

The skilled person will understand that various types of optical systemmay be used to provide the one-to-one correlation between an individuallight detection element and its corresponding individual field of viewwithin the observed scene 506. For example, in embodiments, the opticalsystem may comprise a single lens (as in a camera), or a micro-lensarray where each micro-lens is associated with an individual detector.But any suitable photodetector comprising an array of light sensingelements is possible and may be used for this purpose.

It can be seen that not all of the labelled objects (A, B, C, D) arelocated at the same distance from the light detector 502 as therespective others, in FIG. 5. In this example, object C is closest tothe light detector 502, objects A and D are the next-nearest, at thesame distance from the light detector 502 as one another, and object Bis the furthest from the detector 502. The light detector 502 and/or acontroller that is communicatively coupled to the light detector 502(not shown in FIG. 5) may be configured to account for the differentrespective depths of one or more objects within an observed scene. Thisis discussed for example in GB patent application no. 2002276.0, asfiled by the current applicant, the entirety of which is incorporatedherein by reference. However, the methods disclosed therein are not themain focus of the present disclosure and so are not repeated herein.

FIG. 6 shows a combined system, comprising a holographic projector (orlight source) and a light detector, similar to the holographic projectorand light detector systems shown, respectively, in FIGS. 4 and 5. Theholographic projector comprises an SLM 402 and a projection lens 404.The holographic projector further comprises a Fourier transform lens(not shown) arranged to form a holographic reconstruction in free space(so also not shown) between the SLM 402 and the projection lens 404. Asexplained above, the projection lens 404 forms an image of the‘intermediate’, free space, holographic reconstruction. That image maybe a magnified image, and comprises the structured light pattern of theintermediate holographic reconstruction, projected onto the scene 506.There is also a source of light, upstream of the SLM 402, arranged totransmit light towards the SLM 402, which is not shown in FIG. 6. Thelight may be infra-red (IR) light, visible light or ultra-violet light,dependent on application requirements. In embodiments related to LIDAR,the light source may be infra-red. In embodiments related to head-updisplay, the light source may be visible.

The SLM 402 and projection lens 404 are decentred in FIG. 6. This is toenable the holographic light cone 410, travelling from the projectionlens 404 towards an observed scene 506, to partially overlap with thereflected structured light cone 510, travelling from the scene 506 backtowards the imaging lens 504 and light detector 502.

In FIG. 6, the holographic light cone 410, which has exited the SLM 402and travelled through the projection lens 404, is shown travellingtowards the observed scene 506. The light is described as being‘holographic’ because it comprises light that has been encoded by ahologram on the SLM 402, and thus has formed a structured light pattern,which illuminates the observed scene 506. The light is then reflectedfrom the scene, towards a light detector 502. As described above inrelation to FIG. 4; the SLM 402 may be configured to display a pluralityof holograms, at different respective times. In some arrangements, theSLM 402 may be configured to display a sequence (or series, orplurality) of holograms, one after the other, so that multiple differentstructured light patterns are formed on the observed scene 506, oneafter the other

As was the case in the arrangement of FIG. 5, discussed above, not allof the labelled elements, or ‘objects’, in the observed scene 506 arelocated at the same distance from the light detector 502 as therespective others. Instead, object C is closest to the light detector502, objects A and D are the next-nearest, at the same distance from thelight detector 502 as one another, and object B is the furthest from thelight detector 502. The projection lens 404 is arranged such that thestructured light pattern, which it forms on the observed scene 506, is“in-focus” on each of A, B, C and D at the same time, despite them notbeing co-planar with one another. The holographic light 410 is reflectedby the objects A, B, C and D within the observed scene 506 and theresulting reflected light 510 travels towards the imaging lens 504 andon towards the light detector 502. As described in relation to FIG. 5,above, the light detector 502 in FIG. 6 comprises an array of lightdetection elements, which have a one-to-one correlation with the objects(A, B, C, D) in the observed scene. It will be appreciated that othertypes of scene, with different respective numbers and arrangements ofobjects, and light detectors having a different array of light detectionelements, are also contemplated.

Although not explicitly illustrated in FIG. 6, the light detectionelements of the light detector 502 may be arranged to each output alight response signal, when reflected light is received, at the lightdetector 502, from the observed scene 506. As the skilled reader will beaware; the structured light pattern from the holographic projector maybe ON-OFF gated to create a sequence of “display events”. Optionally,each display event may correspond to a different hologram and thereforeto a different structured light pattern. An ON-OFF gated structuredlight pattern from the holographic projector may give rise to switchingof the light response signals output from the light detectors 502. Thelight response signals may be transmitted to a processor or controller,for use in computation and/or for storage or display purposes. Thus, forexample, a time of flight (TOF) value may be calculated for lighttravelling to and/or from each object (A, B, C, D) within the observedscene 506, based on the light response signal output by thecorresponding light detection element.

The arrangement of FIG. 6 may thus be provided as part of a lightdetection and ranging, “LIDAR”, system, which can be arranged to scan orsurvey a scene.

In holographic projectors, as with many other laser-based applications,safety is an important consideration. The power emitted by a laserwithin a holographic projector system should generally be controlled tobe within a predetermined safety limit, such as an “accessible emissionlimit” (AEL). This is to ensure safe operating levels for the equipmentwithin the system and to ensure eye safety for any users or otherobservers. In certain systems, such as a LIDAR system, for example adirect-view head-up display (HUD), comprised within a vehicle, in whichthe observer (the vehicle driver) effectively looks directly at aspatial light modulator (SLM) that is illuminated by a laser lightsource, the laser may be further controlled to ensure that the driver isnot ‘dazzled’ by the illuminated SLM, and thus can continue to drivesafely.

A potential safety risk for a holographic projector system is the riskof the SLM—which may, for example, be a Liquid Crystal on Silicon (LCOS)SLM, as described above—failing to display a hologram correctly, suchthat the illuminating laser light would not be distributed as intended.For example, if the SLM is an LCOS SLM, and if it erroneously provideduniform phase to all LCOS pixels, prior to application of a softwarelens, then the laser emission would be concentrated into a single spotbecause the lens would focus the uniform illumination to a spot, justlike a physical lens. Such a concentration of laser light could—in someexamples—be damaging to the eyes of the observer. Laser safetyclassification rules therefore typically require engineering controls incase of a ‘scanning system’ failure, such as an LCOS failure of thistype.

The skilled reader will appreciate that, whilst monitoring andcontrolling laser emissions in a holographic projector system isnecessary for safety reasons, in practice there is a demand for it to bebalanced against efficiency and smooth operation of the system, from auser perspective.

There are known laser monitoring techniques, some of which may beapplied to holographic projector systems. For example, WO2018/100395describes a method in which a secondary holographic image is provided,alongside a primary holographic image, wherein the secondary holographicimage does not comprise information intended for the observer of theprimary image, but may be used to obtain a measure of optical power.

The present inventor has recognised that it is possible to accuratelyand efficiently monitor the operation of a holographic projector withoutcausing interruption to its core functionality. In general terms; adetector or detection system may be provided, for detecting aholographic identifier that is located outside an aperture, or otherviewing area, on a holographic replay field on which a holographicreconstruction is formed, when an SLM comprising a hologram isilluminated (or ‘irradiated’) by laser light. The holographic identifiermay be comprised within the zeroth-order holographic replay field, on aholographic replay plane, or it may be comprised within a higher-orderrepeat of the zeroth-order holographic replay field, on the holographicreplay plane. This will be better understood from the detailed examplesbelow.

The holographic identifier may be time-varying, wherein its positionand/or another detected characteristic, and/or a signal generated due tothe detection of part or all of the fingerprint, would be expected tovary in a particular manner, between different respective times, if theSLM was functioning correctly. The detector may comprise or may becommunicatively coupled to a controller, for controlling the laser lightsource, to enable a feedback loop to be implemented. For example, ifdetection of the holographic identifier indicated that the SLM was notbehaving in an expected manner, and thus that there was a risk of thelaser light being transmitted towards the observer in an unsafe manner,the controller could be configured to switch off the laser source or tootherwise block or prevent the laser light from illuminating the SLMfurther, or to reduce the intensity of the laser illumination, at leastuntil any potential malfunction issues had been resolved.

In general terms: each hologram that is to be displayed by an SLM withina holographic projector system is calculated using an algorithm from a“target image”. The “target image” comprises a conventional image suchas, for example, a digital photograph. The present inventor hasrecognised that a fingerprint or identifier may be added to a targetimage—either, for example, in a peripheral area within the target image(for example, in one corner) and/or in an area immediately surroundingthe target image. A hologram may then be calculated of the target image,including the added fingerprint, so that the fingerprint will bereconstructed with the target image, when the hologram is suitablydisplayed and irradiated. The fingerprint that is added to the targetimage may be of any suitable form. For example, it may comprise a simplegeometric pattern such as an array of squares.

One type of holographic projector system that may embody therecognitions made by the present inventor is a holographic LIDAR system.FIGS. 7A and 7B comprises system diagrams for an example of a LIDARsystem that embodies the recognitions made by the present inventor. TheLIDAR system may be provided, for example, in a vehicle, as part of anavigation system, or in a portable device, or in a range of otherapplications.

The system comprises an SLM 754 and a light detector, which in thisexample comprises an array detector 774, which are provided coplanarwith one another but spatially separated from one another, on a commonplane. The SLM 754 is provided in conjunction with a projection lens 756and the detector 774 is provided in conjunction with an imaging lens776. There is a light source which in this example comprises a laserdiode 752. The laser diode 752 is arranged to direct light towards theSLM 754, which is encoded with a hologram and which, upon illuminationof the hologram with light from the laser diode 752, is arranged toreflect structured light towards an image plane 760, via the projectionlens 756. The image plane 760 is located so as to coincide with a targetscene, which is to be observed or interrogated, via the formation of astructured light pattern 758 thereon. The structured light will be atleast partially reflected from the target scene, back towards the arraydetector 774, via the imaging lens 776.

The laser diode 752 is positioned and oriented so that the incominglight arrives at an acute angle to a central lateral axis (not shown) ofthe SLM 754. As a result, the structured light is also reflected awayfrom the SLM 754, via the projection lens 756, at an acute angle,towards the image plane 760.

Although not explicitly shown, the SLM 754 may include a softwarelensing function that enables the image of the holographicreconstruction to be focussed at different respective distances, awayfrom the plane of the projection lens 756. This can accommodate a targetscene having some inherent depth, and thus enable observation of thescene at a plurality of different depths. A plurality of differentlensing functions, each with a different respective focal length, may beprovided, stored in a suitable repository, for selection if/when neededto achieve a desired range for the SLM 754. In other arrangements, theprojection lens 754 is arranged such that fine-tuning of the focus usinga software lens is not necessary.

Although not shown in FIGS. 7A and 7B, an FT (Fourier Transform) lens isprovided in conjunction with the SLM 754. The FT lens may be a softwarelens or a hardware lens or a combination of the two. The FT lens isoperable, with the SLM 754, to form a holographic reconstruction in freespace in a holographic replay field, on a holographic replay plane 741,which is located between the SLM 754 and the projection lens 756. Asdescribed in relation to earlier figures, above, the projection lens 756therefore projects an image of the holographic reconstruction towardsthe image plane 760, which coincides with a target scene that is to beobserved using the LIDAR system 700.

A barrier 742 is located along an optical path of the reflected light,between the SLM 754 and the projection lens 756. The barrier 742 in thisexample comprises substantially first 744 and second 745 walls,positioned either side of a substantially central opening or aperture746. However, other forms of barrier and/or opening are alsocontemplated.

The barrier 742 is arranged to be located on, or substantially on, theholographic replay plane 741 in the example arrangement shown in FIGS.7A and 7B. Moreover, the aperture 746 within the barrier 742 is located,sized and shaped so as to coincide with the zeroth-order holographicreplay field, such that at least part of the zeroth-order holographicreconstruction will be formed, in free space, within the aperture 746.

In other examples, the barrier may be located on a plane other than theholographic replay plane. For example, it may be located on a plane thatis substantially parallel to the holographic replay plane but upstreamor downstream thereof.

The particular size and shape of the aperture 746 may vary betweendifferent respective arrangements. In general terms, the aperture 746may be tailored to the size of the holographic replay field, whichitself depends on a number of factors such as pixel size of SLM 754 andwavelength of the light. The size and shape of the holographic replayfield is not affected by the changes of hologram content betweendifferent respective holograms. However, a holographic replay field maybe translated on its holographic replay plane, using a software grating.Therefore, some tolerance in the size of the aperture may be required,to accommodate any such movements. In other words, the aperture 746 maybe sized and shaped so as to be slightly larger than the holographicreplay field. The skilled reader will know, however, that the magnitudeof any such movements are very small. In some arrangements, the aperture746, and the barrier 742, may be moveable.

In this example, the aperture 746 is substantially quadrilateral as canbe seen better from FIGS. 8A to 8D, which are discussed below—howeverother types of aperture are also contemplated, in respective otheroptical arrangements. The aperture 746 in the example of FIGS. 7A and 7Bis located, sized and shaped so as to occupy most of the zeroth-orderholographic replay field, and to surround a first part, or a firstportion, of the holographic reconstruction, which is intended for onwardtravel towards the projection lens 756, to form an image of that firstportion of the holographic reconstruction on the image plane 760, forobservation of the target scene.

According to the methods described herein, there may be a second part,or second portion, of the holographic reconstruction that is not locatedwithin the aperture 746, and which therefore will be blocked from onwardtransmission by the walls 744, 745 of the barrier 742. That secondportion of the holographic reconstruction may comprise a holographicidentifier, which may be referred to as a holographic ‘fingerprint’. Thefingerprint could take any suitable form. For example, the fingerprintcould be a distribution of light, which changes between differentrespective holograms and/or is moved around as a grating is applied tothe hologram(s) that is/are displayed and illuminated, on the SLM 754.As mentioned above, the fingerprint may derive from a marker, orfingerprint, that was incorporated into a target image before hologramcalculation, and holographically reconstructed with the remainder of thetarget image, upon irradiation of the hologram.

The first and second portions of a holographic reconstruction, mentionedabove, may be referred to as first and second respective light patternregions.

FIG. 7B shows a magnified view of an area 790, within FIG. 7A. As can beseen more clearly from FIG. 7B, one or more light detectors can beprovided, to detect the holographic identifier, or ‘fingerprint’. Forexample, in FIGS. 7A and 7B there are first 748 and second 749photodiodes, provided either side of the aperture 746, respectively onthe first 744 and second 745 walls of the barrier 742. In FIGS. 7A and7B, the photodiodes 748, 749 are provided on the faces of the walls 744,745 that are on the SLM side of the barrier 742. But it will beappreciated that other positions are contemplated for the lightdetectors, in certain arrangements. Moreover, in the example of FIGS. 7Aand 7B there are only two photodiodes shown, but any suitable number andarrangement of light detectors may be provided.

The light detectors, such as the photodiodes 748, 749 of FIGS. 7A and7B, may be configured to detect the presence of the holographicfingerprint, and to transmit detection signals to a suitable processoror controller. This is discussed further, below. Such a processor orcontroller may be communicatively coupled to another processor orcontroller, which controls operation of other aspects of the LIDARsystem 700, including the operation of the laser diode 752. A feedbackloop may be implemented wherein, if the detection signals from thephotodiodes 748, 749 differ from one or more expected signals, whichwould have been detected, if the SLM 754 was correctly displaying thecurrent hologram at a given time, the control aspects of the LIDARsystem may be configured to take action, to prevent or reduce furtherincorrect operation of the SLM 754. For example, the control aspects ofthe LIDAR system 700 may be configured to pause or stop operation of thelaser diode 752, or to reduce its intensity, or to block the path of thelaser light either before it reaches the SLM 754, and/or between the SLM754 and the observer, at least until the SLM 754 can be checked and, ifnecessary, fixed.

When a plurality of photodiodes are provided, the holographicfingerprint may be expected to trigger a detection signal from just one(or just a sub-group, within a larger group) of the photodiodes, at anygiven time. The photodiode(s) that is/are expected to detect a lightsignal, due to the fingerprint, may vary between different respectivetime frames or time instances. This may be, for example, due to theholographic replay field being translated on the holographic replayplane, due to the presence of a software grating. The light spots orother light distribution(s) that make up the fingerprint would, in sucha situation, be translated along with the remainder of the light of theholographic reconstruction, when its holographic replay field istranslated.

In general terms, having a moving fingerprint, which is detectable bydifferent light detection elements at different respective times, mayprovide increased comfort that a holographic projector is functioningproperly, because the safety check does not rely solely on one sensor.Instead, it enables the comparison of detected signals to one or moreexpected signals (or, to one or more expected signal patterns) for eachof a plurality of sensors, over time, and therefore provides a higherdegree of validation. For example, a holographic projector system couldbe arranged to validate (or invalidate) the irradiation of a hologrambased on a number of time-varying signals from each of a plurality oflight sensors, which are dedicated to detecting the fingerprint. Havingredundancy between sensors, and optionally also between signal instancesfor which a comparison is made, increases the overall likelihood that aholographic projector system is configured to accurately determinewhether or not its components, such as its SLM, are functioningproperly. Moreover, for laser safety it may be necessary to shut off orreduce the laser power very quickly, for example, within two grating orhologram changes, in case of SLM failure, to ensure there is no eyehazard to an observer. Having multiple detectors, corresponding tomultiple possible positions of one or more light spots within aholographic reconstruction, is likely to increase the probability ofdetecting a fault within a relatively small number of grating/hologramchanges, and thus increase the speed at which such a failure of the SLMcan be detected and, when possible, remedied.

In the example of FIGS. 7A and 7B, the control aspects of the LIDARsystem 700 are represented as including a system controller 705, ahologram controller 710, and a scene detection controller 720. Thesystem controller 705 is an overall (or ‘central) controller for theLIDAR system 700 and may be configured to receive inputs from, andprovide outputs to, both the hologram controller 710 and the scenedetection controller 720. In FIGS. 7A and 7B, the system controller 705is also configured to receive inputs from the photodiodes 748, 749.There may also be other inputs 730 provided to the system controller705, and/or the system controller 705 may provide one or more otheroutputs 740.

The hologram controller 710 is configured to control the supply ofholograms to the SLM 754, and to control operation of the laser diode752. The scene detection controller 720 is configured to receive lightdetection signals from the array detector 774 and to transmit thosereceived light detection signals, or to transmit determinations made inrelation to those received light detection signals, to the systemcontroller 705, in order for observations to be made about the targetscene, which coincides with the image plane 760. LIDAR techniques forthe observation of such a scene are known and are not the main focus ofthe present disclosure, so will not be discussed in any more detailherein.

Although control aspects such as the system controller 705, hologramcontroller 710, and scene detection controller 720 are shown in FIGS. 7Aand 7B as being physically distinct from one another, this is aschematic/functional representation only. In practice, their functionsmay be carried out, in any suitable combination, by any suitablecomputer(s), or controller(s) or processor(s).

The system controller 705 is configured to control the selection of anappropriate hologram and/or an appropriate grating function and/or anappropriate software lens, for display on the SLM 754. The hologramcontroller 710 and/or the system controller 705 can also conveyoperational signals to the laser diode 752, for example to control thetiming of light pulses towards the SLM 754.

The SLM 754 may be controlled to display different holograms, atdifferent respective times. Each hologram, when irradiated, will giverise to a holographic reconstruction of a distinct structured lightpattern, with discrete areas of light (of a particular brightness) andwith dark areas therebetween. A plurality of different holograms may beindividually displayed at random, or in sequence. In an embodiment, thestructured light pattern projected onto the scene remains the samebetween two holograms but the “fingerprint” is changed between them.This provides timely evidence that the SLM is still operating correctly(due to the changing signals on the photodiode) even when the LiDARsystem does not require a change in the structure light pattern.

According to an example, the SLM 754 may be controlled to display firstand second holograms, one after the other, in which ‘main’ part of thehologram (that represents the target image) is unchanged between thefirst and second holograms, but the holographic fingerprint is differentin each of the first and second holograms. In such an example, thestructured light pattern that is projected onto a scene remains the samebetween two holograms, but the “fingerprint” is changed between them.Therefore, the expected light detection signals would be different foreach of the first and second holograms. This may provide timely evidencethat the SLM is (or is not) still operating correctly, even when theLiDAR system does not require a change in the structured light patternon the scene.

The hologram controller 710 may be configured to either calculate anappropriate hologram or to retrieve an appropriate hologram from amemory, for display on the SLM 754 at a given time. It may comprise anysuitable combination of hardware and software. It may include a memoryand/or it may have access to a separate memory. It may also be comprisedor be communicatively coupled with a data frame generator and a displayengine, which may comprise for example a Field-Programmable Gate Array(FPGA). The display engine may be configured to combine the generated orretrieved hologram with any other suitable aspects, such as a softwarelens and/or a software grating, for display by the SLM 754.

The hologram controller 710 may control (or ‘drive’) the SLM 754 todisplay the appropriate hologram, with a software lens and/or a softwaregrating, if selected. As a result; when the SLM is irradiated by thelaser diode 752, the resultant holographic reconstruction will beformed, at the image plane 760, and should be aligned with an individualfield of view (IFOV) of the array detector 774, for the purpose ofobserving the scene.

As mentioned above; the system of FIGS. 7A and 7B may operate based on afeedback loop, wherein information derived from one or more previousframes may be used to drive subsequent selections and/or other actions.For example, information derived from one or more of the photodiodes748, 749 may be used to control subsequent control of the laser diode752. In particular, the photodiodes 748, 749 may be located so as todetect the presence (or absence) of the holographic fingerprint thatshould be displayed, in the second portion (or, second light patternregion) of the holographic reconstruction that is formed at theholographic replay plane 741, due to illumination of a hologram on theSLM 754, at a given time.

The system controller 705 may be configured to receive one or more lightdetection signals from the photodiodes 748, 749, which indicate one ormore characteristics of the holographic fingerprint, and to compare themto the corresponding one or more characteristics that would have beenexpected for the holographic fingerprint, if the correct hologram wasbeing correctly illuminated, at a given time. A ‘characteristic’, inthis sense, may comprise, for example, a binary indicator as to thepresence or absence of the light of a holographic fingerprint, at one ormore specific sensors. The system controller may therefore, for example,be configured to compare an expectation of which sensors were expectedto detect light of the holographic fingerprint, at a given time, to arecord or measurement of which sensors have, in practice, detected lightof the holographic fingerprint, at that time. The system controller mayalso, or instead, be configured to compare an expected time pattern orsequence of different respective sensors (at different respectivelocations) detecting light of the holographic fingerprint over a periodof time to a record or measurement of which pattern or sequence sensorshave, in practice, detected light of the holographic fingerprint, overthat time period.

In some cases, the characteristic of the holographic fingerprint maycomprise a non-binary (or, ‘greyscale’) indicator, such as a magnitudeor other measurement, derived from a light sensor signal. For example,if a fingerprint comprises more than one light spot, of differentrespective sizes and/or brightnesses, the system controller may beconfigured to determine, from received light sensor signals, whether alight spot of the expected size and/or brightness has been detected (bythe sensor that would have been expected to detect it) at a given time.

The system controller 705 may be programmed, as detailed above, to‘know’ (and possibly to be in control of) which hologram should bedisplayed at a given time—for example, during a particular ‘frame’ ofoperation of the LIDAR system 700. According to the present disclosure,the system controller 705 can also be programmed to ‘know’ (and possiblyto be in control of) which holographic fingerprint should be comprisedwithin the second light pattern region of the zeroth-order holographicreconstruction of that hologram, when it is correctly and safelyilluminated by the laser diode 752. Therefore the system controller705—or, any other suitable processor or controller comprised within, orcommunicatively coupled to, the LIDAR system 700—may be configured tocompare one or more signals from the photodiodes 748, 749 with one ormore corresponding expected signals, based on detection of theholographic fingerprint, at a given time.

The photodiodes may be configured to detect, in relation to theholographic fingerprint, any combination of, for example: the presenceor absence of a fingerprint (i.e. an indicator of whether anyfingerprint has been detected); an indication of the fingerprint'slocation (for example, an indication of which photodiode(s) it has beendetected by); an indication of its brightness, shape, light distributionpattern, or size. As mentioned above; several sensors may be configuredto detect the fingerprint, and in some cases the identity of thesensor(s) that is/are expected to detect the fingerprint will betime-varying, as the fingerprint is translated around the holographicreplay plane.

The fingerprint may have any suitable shape, or light distributionpattern—for example, it could comprise a simple pattern, such as twolight spots, or it could be more complex. For example, the fingerprintcould comprise a pattern or formation in which there is greyscalevariation in light intensity, between different respective spots orother positions, within the fingerprint. Regardless of the complexity ofthe fingerprint, the system controller 705 may be configured to monitorhow (and/or when) the fingerprint (or parts of the fingerprint)interact(s) with the light sensors. In the example of FIGS. 7A and 7B,the system controller 705 can be arranged to know which sensors of aplurality should be activated, and which should not be activated, witheach time frame, based on the positions of the sensors and theshape/configuration of the fingerprint's light pattern, and (ifapplicable) its expected movements, around the holographic replay plane741. Therefore, the system controller 705 can use the outputs of each ofa plurality of sensors—such as the photodiodes 748, 749—to validate orinvalidate the operation of the system, for a given time frame.

If a comparison determines that a detection signal for a fingerprintdoes not match an expectation for that fingerprint, at a given time, atleast to within a predetermined tolerance level, the system controller705 may be configured to take action, to avoid or at least reduce therisk of malfunction of the LIDAR system 700, for example to prevent therisk of the laser dazzling and/or causing eye damage to an observer.

The action that the system controller 705 may be configured to take, inthe event that any detection signal related to a fingerprint does notmatch an expectation, may depend on the details of the signal for whichthere is a mismatch between an expectation and a detection, and possiblyon the nature and/or the extent of the mismatch. The system controller705 may be programmed (or otherwise configured) to follow differentrespective procedures, dependent on which signal(s) (and/orcharacteristic(s)) are found not to match their expectation, and/ordependent on how much they differ from their expectedlevel/location/magnitude etc.

At least in some cases, if the system controller 705 determines that adetection signal relating to a holographic fingerprint does not meet anexpectation, at least to within a predetermined level of tolerance, itmay be configured to issue a signal to stop or at least to pause theoperation of the laser diode 752, or to reduce its power, and/or tootherwise block the path of light between the laser diode 752 and theobserver, via the SLM 754. This is to prevent the risk of the laserlight being incorrectly transmitted by the SLM 754—for example, beingtransmitted as a concentrated spot of light—towards the observer, whichcould cause him or her to suffer eye damage and may also dazzle theobserver, leaving him or her unable to see temporarily. It will beappreciated that the risk of the observer being dazzled may be very highif he or she is the driver of a vehicle, even if the laser light wouldnot be concentrated or powerful enough to cause eye damage.

In some arrangements, the system may be configured so that the light ofthe holographic reconstruction (or, of an image of the holographicreconstruction), which is aligned with an individual field of view(IFOV) of the array detector 774 for the purpose of observing a scene,has an optical power that is within eye-safe limits for each IFOV.Therefore, if the SLM functions correctly, the optical power that isdetected by the array detector 774 and that may be observed by anobserver should not pose the risk of eye damage to the observer.However, in such an arrangement, whilst each individual IFOV may have aneye-safe amount of optical power, the total amount of optical power forall of the IFOV's (or, for several of the IFOV's combined) may exceed(possibly, greatly exceed) eye safe limits. Therefore, if the SLM failedand, for example, all the optical power was focused into a singlecentral spot, the optical power of that spot would be potentially verydangerous to the eyes of an observer. It is therefore very important, insuch an arrangement, to know that the SLM is working correctly to spreadthe light and, thus, the optical power, over the different individualfields of view (IFOV's), within a scene.

A comparison between one or more expected signals and one or moremeasured signals may be carried out at any suitable intervals. Forexample, it could be synchronised with the frame intervals of the SLM754. In some arrangements, the comparison may be done periodically,independently of the SLM frame rate, or, for example, synchronised withthe laser pulse rate, or at any other suitable frequency.

FIGS. 8A to 8D illustrate an example of a holographic reconstructionthat includes a holographic fingerprint, and which may be formed by aholographic projection system, for example a LIDAR system such as theone shown in FIGS. 7A and 7B. It should be appreciated that this exampleis illustrative only and should not be regarded as limiting. In FIG. 8A,a zeroth-order holographic replay field 820 is shown as being asubstantially quadrilateral area. It is comprised on, or within, aholographic replay plane, however the remainder of the holographicreplay plane is not shown in FIG. 8A, for simplicity. The higher-orderholographic replay fields are also not shown in FIG. 8A, however theskilled reader will appreciate that the irradiation of a pixelateddisplay device such as an SLM, for example an LCOS SLM, will lead to thecreation of multiple orders of replay fields, with the zeroth-orderbeing at the centre and being the brightest of the replay fields. Forsimplicity, the zeroth-order holographic replay field 820 will bereferred to as ‘the holographic replay field 820’, in the presentdescription.

An aperture 840 is defined within the holographic replay field 820. Theaperture 840 may be formed as an opening or a window in a barrier, suchas the barrier 742 shown in FIGS. 7A and 7B, or in another suitableformation. However, the walls of such a barrier are not shown in FIG.8A, to facilitate sight of the features that would otherwise be blockedby those walls. The aperture 840 is shaped, sized and located in thisexample so as to frame a first portion—or, a first light patternregion—of a holographic replay field, within which a holographicreconstruction is formed. The holographic reconstruction is created bythe irradiation of a hologram on an SLM, using an optical arrangementsuch as that shown in FIGS. 7A and 7B. The first light pattern regioncomprises a plurality of blocks of light 830, arranged in a gridformation, wherein each block of light 830 occupies its own respectivearea (or co-ordinate) within the grid formation. In this example, thereare 16 blocks of light 830, arranged in a 4×4 grid formation. The blocksof light 830, in their grid formation, form a structured light pattern.The structured light pattern may be translated, or scanned, around theholographic replay plane, which will be discussed in more detail, below.

As per the example detailed above in relation to FIGS. 7A and 7B, theaperture 840 in this example (and the barrier or other formation inwhich the aperture 840 is formed) is configured to allow transmission ofthe structured light pattern that is comprised within the first lightpattern region, i.e. that is formed within the perimeter of the aperture840. Therefore the structured light pattern comprising the grid of 16blocks may be transmitted, for example towards a projection lens, inorder for an image of that structured light pattern to be formed on animage plane, within a scene that is to be observed or interrogated.

There is also a second portion—or, a second light pattern region—withinthe holographic reconstruction in FIG. 8A. The second light patternregion comprises the area, within the holographic replay field 820,which lies outside of the first light pattern region and thus liesoutside the perimeter of the aperture 840. Therefore, in practice, whenthe aperture 840 is formed within a barrier or other formation, thewalls of that barrier (which surround the aperture) would prevent anylight within the second light pattern region from being transmittedonwards, for example towards a projection lens, image plane, or scene orobject that is to be observed. Any light within the second light patternregion would therefore not contribute to the structured light patternthat is incident upon a target scene or object.

The above notwithstanding; the present inventors have recognised thatproviding a light pattern within the second light pattern region—forexample, providing a holographic identifier or fingerprint—can be highlyuseful. This is because such a holographic fingerprint can be detected,and thus the holographic relay field can be monitored, just outside ofthe aperture or area in which the first light pattern region, whichcomprises the portion of the structured light pattern that is to betransmitted on to a target scene or object, is formed. However, theholographic identifier can itself be prevented from transmission towardsa target scene or object of interest, at least in some arrangements.

The holographic identifier, according to this method, can comprise adefinitive fingerprint, which may be time varying, and thecharacteristics of which can be used to make determinations about themanner in which a hologram has been illuminated, within an opticalsystem such as a LIDAR system, and in particular to detect potentialfaults therein.

Suitable detectors, such as light detectors, can be located within asecondary light pattern region of a holographic replay field, to detectand/or to monitor a holographic fingerprint. In FIG. 8a , fourphotodiodes 801, 802, 803, 804 are provided for this purpose. In thisexample, the positioning of the photodiodes 801, 802, 803, 804 has beenselected so as to coincide with multiple different predeterminedpositions of a holographic fingerprint. The photodiodes 801, 802, 803,804 are spatially separated from one another, in two directions (shownas the vertical (y) and horizontal (x) directions, in FIG. 8a ). Thephotodiodes 801, 802, 803, 804 are located just outside the perimeter ofthe aperture 840. They are therefore configured to detect the presenceof an identifier or fingerprint in the secondary light patternregion—not to detect any light within the first light pattern region.

In some arrangements, the photodiodes 801, 802, 803, 804 may beconfigured to detect a magnitude, or a size, or a strength of light,within the holographic fingerprint. In other arrangements, thephotodiodes 801, 802, 803, 804 are configured for binary operation. Insuch an arrangement, the output of each photodiode 801, 802, 803, 804would either be a 0 (no light) or a 1 (light). A combined orconcatenated output may be provided, comprising a combination of theindividual binary readings for the four photodiodes 801, 802, 803, 804.For example, if for a particular frame, the first and second 801, 802photodiodes are expected to detect light of the fingerprint and thethird and fourth 803, 804 photodiodes are not expected to detect lightof the fingerprint, the expected concatenated output would be ‘1100’. Inpractice, if the concatenated output from the four photodiodes 801, 802,803, 804, for that frame, was anything other than ‘1100’, this may beregarded as an indicator of a ‘fail’ within the system, in relation towhich steps may have to be taken, as previously described. The outputprovided by the fingerprint sensors is continually changing in responseto the light pattern on the replay plane.

The holographic fingerprint in this example comprises two relativelysmall blocks of light, offset from one another in two directions (shownas the vertical (y) and horizontal (x) directions, in FIG. 8a ). Forsimplicity, these blocks will be referred to as an ‘upper’ block and a‘lower’ block herein. However, it should be appreciated that theserelative positional terms are used for illustrative purposes only, inrelation to the example shown in FIGS. 8A to 8D, and should not beregarded as limiting. Similarly, any reference to ‘left’, ‘right’ or anyother relative positional terms herein is to aid understanding of theillustrative example shown in FIGS. 8A to 8D, and should not be regardedas limiting.

The holographic reconstruction—which includes the grid of blocks oflight 830, which occupy the first light pattern region, and the twoblocks of light that make up the holographic fingerprint in the secondlight pattern region—is shown in a first position in FIG. 8A. As theskilled reader will know, a software grating can be combined with ahologram on an SLM, wherein a function of the software grating is todetermine, or to move or shift, the position of the holographic replayfield (indeed, of the entire array of replay fields, including higherorders, not shown in FIGS. 8A to 8D) on the holographic replay plane.For convenience, this may be referred to as ‘scanning’ the holographicreplay plane but the person skilled in the art will appreciate thefundamental differences between the holographic, structured light systemdisclosed herein and LIDAR systems that “scan” using e.g. a rotatingprism. The first position of the holographic replay field, and thereforeof the holographic reconstruction, shown in FIG. 8A can thereforecorrespond to a first grating being applied to the hologram or, forexample, to no grating being applied. In this example, there are 16different positions of the holographic reconstruction, eachcorresponding to a different respective grating function, i.e. 4different x-gratings and 4 different y-gratings, resulting in 16different combinations.

When the grid of blocks of light 830, which occupy the first lightpattern region, is in the first position (of sixteen), each block oflight 830 occupies the upper left-hand corner of its respective square,or co-ordinate, within the grid. When the upper and lower blocks oflight, which make up the holographic fingerprint in the second lightpattern region, are in the first position (marked as position 814A inFIG. 8A), the lower block coincides with a first photodiode 801.Therefore, the fingerprint (or at least the lower block, within thefingerprint) is detectable, when the holographic reconstruction is inits first position. The upper block does not coincide with any of thephotodiodes, in this position. An expected light signal from thephotodiodes 801, 802, 803, 804 for this position would therefore be, inbinary form, ‘1000’.

When the holographic replay field (and holographic reconstructions)shown in FIG. 8A is comprised within a LIDAR system, the holographicreconstruction (or an image of it) is projected onto a target object orscene, and a light detector is arranged to detect light reflected fromthe target or scene. In embodiments, each portion of the target or scenethat is illuminated by the light within each respective square, orco-ordinate, within the grid of the holographic replay field in FIG. 8A,is detected by a respective sensor, within a plurality of sensorsarranged in a regular array in a LIDAR detector. When each light spot830 within the grid is scanned (using a software grating) over itsrespective square, or co-ordinate, only one sensor per square, orco-ordinate, within the LIDAR detector will receive light at any onetime, and the other sensors of each square or detector will be dark,thus generating no detection signal.

FIGS. 8B to 8D show the holographic reconstruction of FIG. 8A in threefurther possible positions, corresponding to three further respectivegrating functions.

In FIG. 8B, which corresponds to a fourth (of sixteen) grating function,each block of light 830 in the first light pattern region occupies theupper right-hand corner of its respective square, or co-ordinate, withinthe grid. In the second light pattern region in that fourth position(marked as position 814B in FIG. 8B), the lower block of the holographicfingerprint coincides with a second, different photodiode 802.Therefore, the fingerprint (or at least the lower block, within thefingerprint) is detectable, when the holographic reconstruction is inits fourth position. The upper block does not coincide with any of thephotodiodes, in this position. An expected light signal from thephotodiodes 801, 802, 803, 804 for this position would therefore be, inbinary form, ‘0100’.

In FIG. 8C, which corresponds to a thirteenth (of sixteen) gratingfunction, each block of light 830 in the first light pattern regionoccupies the lower left-hand corner of its respective square, orco-ordinate, within the grid. In the second light pattern region in thatthirteenth position (marked as position 814C in FIG. 8C), the upperblock of the holographic fingerprint coincides with a third photodiode803. Therefore, the fingerprint (or at least the upper block, within thefingerprint) is detectable, when the holographic reconstruction is inits thirteenth position. The lower block does not coincide with any ofthe photodiodes, in this position. An expected light signal from thephotodiodes 801, 802, 803, 804 for this position would therefore be, inbinary form, ‘0010’.

In FIG. 8D, which corresponds to a sixteenth (of sixteen) gratingfunction, each block of light 830 in the first light pattern regionoccupies the lower right-hand corner of its respective square, orco-ordinate, within the grid. In the second light pattern region in thatsixteenth position (marked as position 814D in FIG. 8D), the upper blockcoincides with a fourth photodiode 804. Therefore, the fingerprint (orat least the upper block, within the fingerprint) is detectable, whenthe holographic reconstruction is in its sixteenth position. The lowerblock does not coincide with any of the photodiodes, in this position.An expected light signal from the photodiodes 801, 802, 803, 804 forthis position would therefore be, in binary form, ‘0001’.

An SLM may be configured, or controlled, to apply grating functions to ahologram on a cyclical basis, to repeatedly move the correspondingholographic reconstruction through its sixteen possible positions. AnSLM may be configured, or controlled, to dynamically display a pluralityof different holograms and, for at least some of those holograms, toapply one or more grating functions in order to change the position ofthe corresponding holographic reconstruction(s) on a holographic replayplane, on a dynamic basis.

The controller of a holographic system, such as the system controller705 of the LIDAR system 700 of FIGS. 7A and 7B, or any other suitablecontroller, may be configured to ‘know’ which hologram and, whenapplicable, which grating and/or which software lens should be displayedand illuminated on an SLM, at a particular time. Such a controller maybe configured to ‘know’ what fingerprint to expect, and what the signalsdetecting light of that fingerprint should be. For example, thecontroller may be programmed to know what position some or all of aholographic fingerprint should be in, at a given time, in absolute termsand/or relative to the positions of the photodiodes 801, 802, 803, 804.In the example of FIGS. 8A to 8D, this means that the system controllershould know which photodiode, if any, should be illuminated by one orother of the blocks of the holographic fingerprint, at a particulartime—for example, during a particular frame of operation of the SLM. Thesystem controller 705 may therefore be configured to validate thereceived detection signals from the photodiodes 801, 802, 803, 804, ifthey match one or more expected detection signals, or an expected timesequence of time signals.

The skilled reader will appreciate that there will be times (forexample, corresponding to particular grating functions and therefore toparticular positions of a holographic reconstruction on the replayplane) at which the holographic fingerprint may be expected not tocoincide with any of the photodiodes 801, 802, 803, 804, in the exampleof FIGS. 8A to 8D. At such times, an expected binary output from thefour photodiodes 801, 802, 803, 804 would be ‘0000’. This is illustratedin FIG. 8E herein, which shows signal traces 861, 862, 863, 864 whichcorrespond respectively to the expected light detection signals fromeach of the first to fourth photodiodes 801, 802, 803, 804, over time.The signal traces 861, 862, 863, 864 are shown over one time period 852,during which the holographic reconstruction cycles through its sixteendifferent positions. In this example, the system controller may beconfigured to compare a received (i.e. measured, in practice) signaltrace, during such a period 852, from one or more of the photodiodes801, 802, 803, 804 to the expected signal trace(s) shown in FIG. 8E. Ifthere is a mismatch in the timing and/or in the strength of the signalreceived, form any of the photodiodes 801, 802, 803, 804, during thattime period 852, the controller may take that as an indication that anincorrect hologram has been illuminated and/or that a hologram has beenilluminated incorrectly, for example due to a fault with the SLM orSLM-driver which means that the right hologram is not displayed (or thatno hologram is displayed at all). This may therefore serve as a triggerfor the controller (or for a user) investigating potential problemsfurther. It may be taken as a trigger for the laser light source, whichis illuminating the SLM, to pause or stop its operation, or to haltemission of light, between the laser light source and an observer, inanother way such as via activating a physical barrier or shutter, toeliminate any further possibility of malfunction that could risk usereye damage and/or dazzling an observer.

It will be appreciated that the scanning order of the gratings does notneed to follow the “left-to-right, top-to-bottom” sequence that isdescribed above. For example, an order of the gratings could beconfigured so that there is minimal deadtime in the fingerprint—i.e.minimal time when the expected output of the photodiodes would be‘0000’. In such an example, there would therefore always be someevidence of whether the fingerprint is showing a correct signal, becausethere would always be at least one detected photodiode signal, tocompare to a detected signal.

By way of summary, FIG. 9 illustrates a safeguarding method, which canbe adopted by a LIDAR system controller or other suitable controller orprocessor, in accordance with the recognitions made by the presentinventors herein. The method 900 comprises a feedback loop, wherein thesteps of the method 900 can occur on a cyclical basis. Those steps areas follows.

At step 902, a light source, such as the laser diode 752 of the LIDARsystem 700 of FIGS. 7A and 7B, emits light, for example laser light. Thelight is directed towards a display device, such as a pixelated displaydevice, for example an SLM, for example an LCOS SLM. The laser lightsource may emit light under the control of a suitable controller.

At step 904, the display device is illuminated. The display device,which might be an LCOS SLM, is configured to display a hologram, whichmay be combined with a grating and/or a software lens. An FT lens mayalso be provided, in conjunction with the LCOS SLM, in order for itsillumination to give rise to the formation of a holographicreconstruction.

At step 906, a detector, for example a light detector such as aphotodiode (or photodiodes), detects a light signal within theholographic reconstruction that is formed by illumination of the displaydevice. Although the examples discussed thus far in the presentapplication have the photodiode situated at the zero order holographicreplay field, on which a holographic reconstruction is formed, it ispossible—as will be described in more detail below, in relation tosubsequent examples—for the photodiode to be situated at an image plane,on which an image of a holographic reconstruction is formed and/or to besituated within a higher-order holographic replay field. It is alsopossible for the photodiode(s) to be situated at an interim position,for example between the SLM and the holographic replay plane or betweenthe holographic replay field and a downstream image plane. There may bemore than one detector and it/they may be located, sized and/or shapedin order to detect a particular feature of a holographic reconstruction,such as a particular holographic identifier or fingerprint.

At step 908, one or more signals from the detector(s) is received andprocessed by a controller—which comprises, or which is comprised within,or which is communicatively coupled to, the controller that controlsemission of light from the laser light source at step 902. Thecontroller at step 908 processes the signal(s) from the detector(s) todetermine whether they are as expected. If they are, the controller canverify holographic identifier or fingerprint.

If the holographic identifier or fingerprint has been verified by thecontroller, at step 910 the controller (or another controller,communicatively coupled thereto) proceeds to issue an instruction to thelaser to emit light (or to continue emitting light), at which point themethod 900 returns to step 902 and repeats itself. If, however, theholographic identifier or fingerprint has not been verified at step 908,the safeguarding method will pause, or stop, the light emission, orreduce the optical power of the light emission, to enable investigationsto occur, and for any appropriate fixes to be made, in order to ensuresafe operation of the system.

According to another example, which also embodies the safeguardingmethod 900 as summarised in FIG. 9, one or more light detectors may beprovided in a higher order holographic replay field, in order to assesswhether a holographic system, such as a LIDAR system, is functioningcorrectly.

FIG. 10A shows cross-sections of the light cones 1008, 1010 that form,respectively, the zeroth-order holographic replay field and one instanceof the first-order (i.e. the negative first-order in the x-direction, or(0, −1) order) holographic replay field, which occur when an SLMdisplaying a hologram is illuminated with suitable laser light. Forsimplicity, the light source and SLM are not themselves shown in FIG.10A.

A holographic replay plane 1014 is depicted by a dashed line, across theupper edges of the cross-sections of the light cones 1008, 1010, in FIG.10A. The holographic replay plane 1014 is the plane at which the(intermediate) holographic replay fields are located, and thus the planeat which a plurality of holographic reconstructions (zeroth-order andhigher orders) of an illuminated hologram will be formed.

The breadth (i.e. the lateral extent, along the x axis) of thezeroth-order holographic replay field is depicted by a double-sidedarrow 1006, in FIG. 10A. A formation 1002, such as a barrier, isprovided just downstream of the holographic replay plane 1014, in thedirection of travel of the light, which is indicated by an arrow 1004.The light is travelling from the illuminated SLM (not shown), towards ascene (also not shown) that is to be observed, for example as part of aLIDAR system. The barrier 1002 includes an opening, or aperture, whichis sized and shaped to match the size and shape of some or all of thezeroth-order holographic replay field 1006. In some arrangements, onlypart of the zeroth-order holographic replay field 1006 may be desiredbecause respectively other parts are known to contain noise (not imagecontent). For example, such noise may result from a hologram calculationalgorithm. Any such noise may therefore be deliberately cropped out, bythe aperture. Thus, the aperture is configured to transmit all lightwithin the zeroth-order holographic reconstruction (or, all of thedesired light within the zeroth-order holographic reconstruction),onwards towards the scene that is to be observed.

The above being the case; it may be disadvantageous for a light detectorto be positioned to detect light in a portion of the zeroth-orderholographic reconstruction, because it would obstruct the path of thezeroth-order holographic reconstruction, towards the scene. Therefore,in this example, one or more light detectors 1012A is provided tomonitor a higher order holographic replay field—in this case, thenegative first-order in the x-direction, or (0, −1) order, replay field.

The methodology in this example is highly similar to that which has beendescribed above in relation to monitoring a portion of a zeroth-orderholographic reconstruction. In short; one or more suitably placed lightdetectors 1012A may be used to monitor the negative first-orderholographic replay field signals—more specifically, to monitor for lightdetection signals relating to a secondary light pattern region, withinthe (0, −1) replay field, which comprises a holographic fingerprint. Forexample, the timings and/or the intensities of such signals may bemonitored, as well as the timings and/or the order at/in whichparticular individual (or sub-groups of) detectors 1012A detect light ofthe holographic fingerprint. Such monitoring may be performed in orderto provide evidence for assessing whether the system—and, in particular,the SLM—is functioning correctly. The light detectors 1012A, or acontroller coupled thereto, can monitor for a change in a signal fromone or more of the detectors 1012A, for example when a grating isapplied to move the holographic replay fields around the holographicreplay plane, and to detect any anomalies between an expected behaviourand a detected behaviour of the system.

In the example of FIG. 10A, light detectors 1012A are provided acrossthe full breadth of the first-order (0, −1) holographic replay field.Therefore, the first-order holographic reconstruction can be detected inits entirety, by the detectors 1012A.

FIG. 10B shows an alternative arrangement, which is highly similar tothe arrangement of FIG. 10A, except that light detectors 1012B areprovided across only part of the first-order (0, −1) holographic replayfield. Therefore, only part of the first-order holographicreconstruction can be detected, by the detectors 1012B. However, thismay be sufficient, in many cases, to provide reliable monitoring of thefunctioning of the system.

FIG. 10C shows another alternative arrangement, which is highly similarto the arrangements of FIGS. 10A and 10B, except that light detectors1012C are arranged along a plane 1011 that extends (along the y axis)between the SLM (not shown) and the holographic replay plane 1014,substantially perpendicular to the holographic replay plane 1014, withinthe light cone 1010 that forms the negative first-order (0, −1)holographic replay field. Therefore, the first-order holographicreconstruction will not be focused on the light detectors 1012C in FIG.10C. Nonetheless, the light at the positions of those light detectors1012C may nonetheless act as a fingerprint, or identifier, for theilluminated hologram. The light signals from those light detectors 1012Cmay therefore be sufficient, in many cases, to provide reliablemonitoring of the functioning of the system.

As the skilled reader will be aware, the first-order holographicreconstruction that is formed upon illumination of a hologram isfundamentally a reproduction of the zeroth-order holographicreconstruction. Therefore, the first-order holographic reconstruction isgenerally highly reliable, as an identifier or fingerprint representingthe zeroth-order holographic reconstruction, and as an indicator as towhether a specific hologram has been correctly illuminated. Theintensity of a first-order holographic reconstruction is reduced by asinc2 envelope, as compared to a zeroth-order holographicreconstruction. Therefore, the sensitivity of one or more lightdetectors, for detecting light signals in a first-order holographicreconstruction, may have to be relatively high.

Thus, in some embodiments, a separate marker, or fingerprint, is notintroduced into a target image, before a corresponding hologram iscalculated. Instead, a higher-order replica of part of the main image(i.e. a higher-order replica of part of the first light pattern region)is used as the fingerprint. For example, the main image, comprisedwithin the first light pattern region, could be the array of light spotsshown in FIGS. 8A to 8D. A selection may be made as to what part of aparticular higher order replay field should be monitored, and lightdetectors positioned accordingly. For example, the photodiodes 801, 802,803, 804 could be aligned with the higher-order replica of the top row(or top two rows) of light spots in the (0, −1) replay field. The systemcould then monitor signals from the light detectors, as compared to oneor more expectations, as detailed above in relation to otherembodiments.

Making use of (part of) a higher order replay field, to act as aholographic fingerprint, is advantageous because it means that adedicated fingerprint does not actually need to be incorporated to themain image. It therefore simplifies the method described herein. It isalso optically efficient because adding a fingerprint inherentlyrequires some of the available light, whereas higher-order replay fieldsare inherently generated when a hologram (or a grating) on a pixelateddisplay device is irradiated. Therefore, if first-order light is usedfor monitoring a holographic reconstruction, no optical power is wasted.Such monitoring can therefore be described as a zero power loss method.

As the skilled reader will be aware, there are four differentfirst-order holographic reconstructions, for each zeroth-orderholographic reconstruction—a negative and a positive, in each of twodirections (e.g. the x and y directions). The four different first-orderholographic reconstructions may have different respective intensities,depending on optical settings of the system in which an SLM comprised. Acontroller or processor may therefore be configured to determine whichof the four first-order holographic replay fields a detector (or aplurality of detectors) should be positioned in, in order to monitor thelight signals therein. In embodiments, the controller is arranged tomonitor whichever higher-order replay field is, on average, thebrightest first t order replay field for all grating positions.

Waveguide Pupil/Viewing Window Expander

The methods and systems described herein can be implemented in a varietyof different holographic projection systems, for example in LIDARsystems that form part of vehicle navigation systems. An example of onetype of holographic projection system, which will be known to theskilled reader, is a direct view head-up display (HUD) system. In such asystem, the optics are configured so that the driver (i.e. the observer)effectively looks directly at the SLM, without a diffuser therebetween.There is therefore a safety imperative to ensure that the SLM functionscorrectly, to avoid causing the driver eye damage and also to avoid‘dazzling’ him or her, with too much light at a given time, which mightimpair his or her ability to drive.

In holographic projection systems such as head-up displays (HUDs) it isdesirable to expand the exit pupil corresponding to the eye box regionor viewing window. In particular, the viewer needs to be able to movehis or her head around and so able to see the complete image from anyposition within a limited area at the eye box/viewing distance. This isknown as the eye motion box (EMB), eye-box or, more generally, viewingwindow. Thus, a pupil expander may be employed to enlarge the EMB orviewing window. Typically, the pupil expander enlarges the EMB bygenerating extra rays by division of the amplitude of the incidentwavefront.

FIG. 11 illustrates an example pupil expander comprising a waveguide. Inthis example, the waveguide comprises two reflective surfaces but thedescription that follows is equally applicable to a slab configurationin which light is guided inside by the slab by internal reflectionsbetween the top and bottom surface of the slab. The general principle ofa waveguide is known in the art and not described in detail herein. Awaveguide guides light within a layer between a pair of parallelreflective surfaces by internal reflection. A pupil expander is formedfrom a waveguide comprising a first graded/partially reflective surface1120 (e.g. a graded mirror having varying reflectivity with distance)and a second fully reflective surface 1110 (e.g. a mirror havingsubstantially 100% reflectivity). In particular, first reflectivesurface 1120 comprises a reflective coating the reflectivity of whichdecreases along the length of the slab. The layer may be glass orPerspex. The waveguide may therefore be a glass or Perspex block orslab. The first reflective surface may be a first surface of the glassblock and the second reflective surface may be a second surface of theglass block, wherein the first surface is opposite and parallel to thesecond surface. Alternatively, the layer may be air and the first andsecond reflective surface may be separate components—e.g. a first andsecond mirrors spatially-separated to form an air gap within which lightpropagates by internal reflection.

Accordingly, as shown in FIG. 11, an input light beam 1102 (which maycomprise spatially modulated light encoded with a picture (i.e. light ofa picture/image or, simply a picture) or spatially modulated lightencoded with a hologram) comprising input light rays enters thewaveguide through an input port thereof. The waveguide is arranged toguide light received at the input port to a viewing window. In theillustrated arrangement, the input port comprises a gap in the firstpartially reflective surface 1120 near one end of the waveguide, butother positions for the input port are possible. The viewing window isan area or volume within which a viewer may view an image as describedherein. The angle of incidence of the input light beam 1102 is such thatthe light rays propagate along the length of the waveguide due tointernal reflection by first partially reflective surface 1120 andsecond fully reflective surface 1110. Example rays are illustrated inFIG. 11. Due to the graded reflectivity of first reflective surface1120, a proportion of light is transmitted by first reflective surface1120 to provide a plurality of output light rays 1104 a-f (herein called“replicas” because they replicate the input light rays) along the lengthof the waveguide. Thus, first reflective surface 1120 forms a viewingsurface. It is said that the pupil (or viewing window) is expanded bythe replicas formed by the waveguide. In particular, by forming aplurality of replicas 1104 a-f along the length of the waveguide, theviewing window is increased in size. Each replica 1104 a-f correspondsto a proportion of the amplitude (intensity or brightness) of the inputlight beam 1102. It is desirable that the grading provides a decrease inreflectivity (or conversely an increase in transmissivity) of the firstreflective surface 1120 along the length of the waveguide such that eachreplica 1104 a-f has substantially the same amplitude. Thus, a viewerhaving a right viewer eye 1130R and left viewer eye 1130L at the eye boxat a viewing distance from the first reflective surface 1120 is able tosee the image at any position within an expanded viewing window, asillustrated by arrows 1140.

The waveguide shown in FIG. 11 expands the viewing window in onedimension—corresponding to the lengthwise direction along which thelight beam propagates within the waveguide—as shown by arrows 1140. Asthe skilled person will appreciate, it is possible to expand the viewingwindow in two dimensions, when required, by using two orthogonalwaveguides.

The first reflective surface 1120 of the waveguide may be coated with acoating comprising a large number of thin films (e.g. 25 or more thinfilms) in order to provide the necessary graded reflectivity. Inparticular, as described above, such thin films or similar coatings needto provide decreasing reflectivity, and thus increasing transmissivity,with propagation distance such that the brightness (ray intensity) ofeach replica 1104 a-f is substantially constant. The amplitude of thepropagating light beam reduces with propagation distance due to outputof the replicas 1104 a-f and due to any other optical losses such asimperfect reflections from the second reflective surface 1110. Thus, thegrading of the first reflective surface 1120 is designed to take intoaccount the drop in intensity of the propagating light beam withpropagation distance, whilst ensuring that each replica 1104 a-f hassubstantially the same intensity so that the image seen has uniformbrightness throughout the viewing window (i.e. at all viewingpositions).

According to the methods described herein, one or more of the replicasin a waveguide—such as the plurality of output light rays 1104 a-f shownin FIG. 11 herein—may be used for monitoring the light within aholographic reconstruction, for example for detecting the presenceand/or one or more characteristics of a holographic fingerprint, and/orfor monitoring detection signals that indicate the presence or absenceof such a fingerprint. Thus, the replicas may be used not only to expandthe viewing window, but also to monitor for correct operation of thedisplay device, such as an SLM, to protect the observer from eye damageand from being dazzled.

One or more light detectors may be provided at a suitable location, formonitoring the light without interfering with the core functionality ofa waveguide. For example, in the example shown in FIG. 11, one or morephotodiodes may be positioned at or near the first reflective surface1120, at a point at which one of the replica rays is expected to contactit. Although not shown in FIG. 11, an additional replica ray may beprovided, which does not travel through the first reflective surface1120 in order to expand the viewing window, but is instead substantiallyabsorbed (or deflected or otherwise interrupted) by a light detector.For example, such an additional replica might be included following onimmediately after replica 1104 f, as shown in FIG. 11. A feedback loopmay be implemented, whereby the signals output by the one or moredetectors in a waveguide may be used to control subsequent operation ofthe laser source, as described in detail above in relation to earlierfigures.

In some arrangements of a waveguide-HUD, for example the arrangement inFIG. 14, discussed in detail herebelow, the holographic reconstructionis not formed until it reaches the retina of an observer's eye. That is,the observer's eye lens acts as the Fourier lens for forming theholographic reconstruction. In such an arrangement, a Fourier lens maybe included, to act only on the replica ray that is used for monitoring.The replica ray therefore may be extracted from the waveguide,propagated through a Fourier Lens and may then travel on towards one ormore monitoring photodiodes.

FIG. 12 shows a slab waveguide 1200 comprising an input port 1201arranged to receive input light 1210 such as light of a picture or lightof a hologram. The slab is made from a material having a refractiveindex greater than air. Light received into the slab 1200 is guided by aseries of internal reflections between a bottom surface 1203 b and anopposing top surface 1203 a. The bottom surface 1203 b may be asubstantially perfect reflector—such as a mirror—and the top surface1203 a may be mostly-reflective. The top surface 1203 a may allow sometransmission of light. Accordingly, light generally propagates along theslab by internal reflection but a series of replicas, R0 to R7, of thelight rays are formed owing to the partial transmissivity of the topsurface 1203 a. The division of light (or replica of the light rays)shown in FIG. 12 functions to expand the exit pupil of the waveguide.Pupil expansion achieved by the light ray replicas allows a viewer,having a right eye 1230R and left eye 1230L, to move (as shown by arrows1240) within a viewing window area (or volume) whilst still receivinglight of the picture—i.e. whilst still be able to see the picture, orhologram. As described with reference to FIG. 11, the reflectivity ofthe top surface decreases with distance from the input port so that theintensity of each replica, Ro to R7, is substantially the same. Theso-called graded-reflectivity of the top surface 1203 a may be providedby a multilayer, dielectric coating. In practice, it is difficult tofabricate an adequate dielectric coating for high qualitydisplay—particularly, full colour display.

As detailed above in relation to FIG. 11, according to the methodsdescribed herein, one or more light detectors may be included in thewaveguide of FIG. 12. For example, an additional replica ray may beprovided, which has the purpose of providing a light signal to aphotodiode or other detector, in order to monitor the operation of theSLM, and ensure eye safety and comfort for the observer.

The present disclosure also provides an improved waveguide based on aslab. For the avoidance of doubt, FIGS. 13 and 14, which illustrateexample system configurations in accordance with this disclosure, show awaveguide formed by two mirrors—rather than a slab with reflectivecoatings—by way of example only. The effects of light refraction are notfully illustrated in the Figures to preserve simplicity but they will bewell-understood by the person skilled in the art.

First Example System

FIG. 13 shows a holographic display system comprising a waveguideforming a waveguide pupil expander in accordance with a first examplesystem configuration. FIGS. 13 and 14 refer to colour projection systemsby way of example only and the present disclosure is equally applicableto a monochromatic system.

The holographic display device comprises a picture generating unitarranged to form a first picture (also called “first image”) and asecond picture (also called “second image”). A first single colourchannel (also called “first display channel”) is arranged to form thefirst picture and comprises a first light source 1310, a firstcollimating lens 1312 and a first dichroic mirror 1314. First dichroicmirror 1314 is arranged to reflect light of a first wavelength along acommon optical path so as to illuminate a spatial light modulator (SLM)1340. The first wavelength of light corresponds to the first displaychannel of a first colour (e.g. red). A second single colour channel(also called “second display channel”) is arranged to form the secondpicture and comprises a second light source 1320, a second collimatinglens 1322 and a second mirror 1324. Second mirror 1324 is arranged toreflect light of a second wavelength along the common optical path so asto illuminate the SLM 1340. The second wavelength of light correspondsto the second single colour channel of a second colour (e.g. green). Inother embodiments, the picture generating unit may comprises a thirdsingle colour/display channel (equivalent to the first and secondchannels) arranged to form a third picture, wherein the third colourchannel corresponds to a wavelength of light of a third colour (e.g.blue). In the illustrated embodiment, SLM 1340 comprises a single arrayof light modulating pixels (e.g. LCOS) that is illuminated by light ofboth the first and second wavelengths. In other embodiments, SLM 1340may comprise separate arrays of light modulating pixels that areilluminated by light of the respective first and second wavelengths.

Holographic display device further comprises a holographic controller1302 arranged to control the picture generating unit. First spatiallymodulated light of the first colour corresponding to the first pictureis output by SLM 1340 to form a first single colour image (e.g. redimage) on a light receiving surface 1370, such as a screen or diffuser.A first single colour computer-generated hologram is calculated by aholographic controller 1302 and encoded on SLM 1340, for example by adisplay driver 1342. The SLM 1340 displays the first hologram and isilluminated by light of the first colour from the first colour/displaychannel to form a first holographic reconstruction on the lightreceiving surface 1370 which is positioned at the replay plane.Similarly, second spatially modulated light of the second colourcorresponding to the second picture is output by SLM 1340 to form asecond single colour image (e.g. green image) on the light receivingsurface 1370. A second single colour computer-generated hologram isencoded on SLM 1340 by holographic controller 1302. The SLM 1340displays the second hologram and is illuminated by light of the secondcolour from the second colour/display channel to form a secondholographic reconstruction on the light receiving surface at the replayplane.

In the illustrated arrangement, a beam splitter cube 1330 is arranged toseparate input light to SLM 1340 and spatially modulated light output bySLM 1340. A Fourier lens 1350 and mirror 1360 are provided in theoptical path of the output spatially modulated light to light receivingsurface 1370. It may be said that a first/second picture is formed onthe light receiving surface 1370. The first/second pictures arefirst/second holographic reconstructions of the respective first/secondholograms. Thus, a composite colour picture may be formed on lightreceiving surface 1370 combining the first and second pictures. Aprojection lens 1380 is arranged to project the first and secondpictures formed on the light receiving surface 1372 to an input port ofa pupil expander in the form of a waveguide 1390. A viewer 1308 may viewa magnified image of the pictures from the expanded eye box—the “viewingwindow”—formed by waveguide 1390 owing to optical power of projectionlens 1380. Waveguide 1390 comprises an optically transparent mediumseparated by first and second reflective surfaces as described abovewith reference to FIG. 11. Thus, holographic display device has an“indirect view” configuration—that is the viewer does not directly viewthe holographic reconstruction, but rather views pictures formed on thelight receiving surface 1370.

In other example implementations, three or more display channels may beprovided configured to display respective single colour holograms. Forexample, a full-colour composite image/picture may be formed bydisplaying respective red, green and blue single colour holograms. Thepresent disclosure may be implemented using a picture generating unitcomprising any number of single colour channels including just onecolour channel.

Second Example System

FIG. 14 shows a holographic display system comprising waveguide pupilexpander in accordance with second example system configuration.

The holographic display system illustrated in FIG. 14 is similar to theholographic display system of FIG. 13 but characterised by the absenceof a screen between the spatial light modulator and viewing plane.Components in FIG. 14 that are similar to those in FIG. 13 have similarreference numerals, but beginning with ‘14’ instead of ‘13’. A firstdisplay channel is arranged to form a first image (e.g. red image) on alight receiving surface, which is positioned at the replay plane. Afirst single colour computer-generated hologram is encoded on SLM 1440by a holographic controller 1402. The SLM 1440 displays the firsthologram and is illuminated by light from the first colour channel toform a first holographic reconstruction on the light receiving surface.Similarly, the second display channel is arranged to form the secondimage (e.g. green image) on the light receiving surface. A second singlecolour computer-generated hologram is encoded on SLM 1440 by holographiccontroller 1402. The SLM 1440 displays the second hologram and isilluminated by light from the second colour channel to form a secondholographic reconstruction on the light receiving surface at the replayplane.

The holographic display device further comprises a beam splitter cube1430, arranged to separate input light to and output light from SLM1440. However, in contrast FIG. 13, the holographic display device is adirect view system. In the illustrated arrangement, a lens 1450 ispositioned in the optical path of the spatially modulated light outputby SLM 1440. Lens 1450 is optional. A viewer 1408 may directly-view thespatially modulated light from the spatial light modulator. In someembodiments, as described above, the lens of the viewer's eye forms aholographic reconstruction on the retina of the eye. In theseembodiments, it may be said that the viewer receives spatially modulatedlight encoded with the hologram. In other embodiments, the viewerreceives light of the picture or light encoded with the picture. Thepicture may be formed at an intermediate plane in free space. Waveguide1490 comprises an optically transparent medium separated by first andsecond reflective surfaces as described above. Thus, the holographicdisplay device has an “direct view” configuration, wherein the viewerlooks directly at the display device (i.e. spatial light modulator),such that the light receiving surface of FIG. 13 is optional.

Again, the arrangements of FIGS. 13 and 14 can be configured, accordingto the methods described herein, to include one or more light detectors,within a waveguide, for detecting the light emitted from an SLM, such asthe SLM 1340 and/or the SLM 1440. For example, the waveguide 1390 or thewaveguide 1490 may be configured to include one or more photodiodes, tomonitor the presence or absence of, and optionally one or morecharacteristics of, one or more replica rays. The waveguide 1390, 1490may be configured to provide an additional replica ray or rays—inaddition to those shown in FIGS. 13 and 14 and describedhereabove—wherein that additional ray or rays is/are dedicated to beingdetected by one or more suitable light detectors, for monitoring theoutput of the SLM 1340, 1440, for ensuring safe operation of therespective system.

Although specific examples have been illustrated and described in detailhereabove, other variations are also contemplated. For example, in FIGS.7A and 7B the aperture 746 and photodiodes 748, 749 are shown anddescribed as being located substantially at the intermediate holographicreplay plane, at which an initial (or ‘original’, or ‘intermediate’)holographic reconstruction is formed, in free space. However, accordingto some arrangements, the aperture and photodiodes may instead belocated at an image plane, with a physical lens (similar to the imaginglens 756 shown in FIGS. 7A and 7B) optionally being present between theSLM and the image plane. There may be a physical optical componentpresent at the image plane, such as a diffuser.

According to some arrangements, a physical lens may be included, to movethe holographic replay plane in the z direction. Therefore, theholographic replay plane may not be located between the SLM and animaging lens, as shown in FIGS. 7A and 7B herein.

In other arrangements, one or more detectors may be implemented in orderto image and/or to monitor the (distribution of) scatter of structuredlight comprised within the zeroth-order replay field of a holographicreconstruction, off the inside surface of projection lens, such as theprojection lens 756, shown in FIGS. 7A and 7B. Such detectors maymonitor for evidence of “safe” distribution of light, as an indicator ofthe safety or operation of the SLM, the irradiation of which has formedthe holographic reconstruction.

One or more light detectors, or photodiodes, which are used to monitorlight to determine safe operation of an SLM may also have otherfunctions. For example, although the examples detailed above monitoringwhether an SLM is displaying correct content, one or more of the lightdetection elements could also be used to monitor the emitted power fromthe laser light source. This can enable the safeguarding method, asdescribed herein, to monitor both that the power of the laser is below athreshold and that the SLM is distributing the light correctly.

For example, in some arrangements it may be possible to use one or morephotodiode measurements as a time trigger for a LIDAR time-of-flightmeasurement. That is; since the photodiodes would be located on the“transmit” path of light emitted (for example, reflected) from an SLM,the time signatures of signals recorded by the photodiodes may be usedto “start the clock” that records the time for a pulse of light, whichleave the SLM, to be reflected back from the scene, towards a scenedetector in a LIDAR system. As the skilled person will know; currentlythe trigger to “start the clock” is taken to from the electronics thatdrive the laser pulse rather than attempting to measure the time of theoutgoing light directly. Therefore, using timing from photodiode signalsalong the transmit path may improve the overall accuracy of the LIDARsystem's measurements and observations.

Thus, methods and systems are described herein that provide soughtafter, and often necessary, monitoring of an SLM, for example within aLIDAR system, to ensure it operates safely and correctly, and does notrisk the eye safety or comfort or driving safety of the observer.

The systems and methods described herein can enable highly reliablemonitoring of scene illumination in a LIDAR system. This could beparticularly useful if, for example, a characteristic of a holographicreconstruction (or an image of that holographic reconstruction) isliable to change with ambient temperature. In such a scenario; byaccurately and closely monitoring the actual characteristics of, and/orthe actual detection signals relating to, the holographicreconstruction, a determination can be made as to its nature andtherefore an appropriate selection can be made, in order to set or amenda subsequent illumination pattern, for the system.

The systems and methods described herein can be provided in a simple andrelatively low-cost manner. The inclusion of an aperture and one or morelight detectors can be readily implemented in existing opticalarrangements, and/or in future optical arrangements. Moreover, themonitoring of signals from one or more light detectors, and issuing ofcontrol signals, accordingly, can be readily carried out by existingcontrollers or other processors, without placing undue computationalburden thereon.

Additional Features

Embodiments refer to an electrically-activated LCOS spatial lightmodulator by way of example only. The teachings of the presentdisclosure may equally be implemented on any spatial light modulatorcapable of displaying a computer-generated hologram in accordance withthe present disclosure such as any electrically-activated SLMs,optically-activated SLM, digital micromirror device ormicroelectromechanical device, for example.

In some embodiments, the light source is a laser such as a laser diode.In some embodiments, the detector is a photodetector such as aphotodiode. In some embodiments, a light receiving surface is a diffusersurface or screen such as a diffuser. The holographic projection systemof the present disclosure may be used to provide an improved head-updisplay (HUD) or head-mounted display. In some embodiments, there isprovided a vehicle comprising the holographic projection systeminstalled in the vehicle to provide a HUD. The vehicle may be anautomotive vehicle such as a car, truck, van, lorry, motorcycle, train,airplane, boat, or ship.

Examples describe illuminating the SLM with visible light but theskilled person will understand that the light sources and SLM mayequally be used to direct infrared or ultraviolet light, for example, asdisclosed herein. For example, the skilled person will be aware oftechniques for converting infrared and ultraviolet light into visiblelight for the purpose of providing the information to a user. Forexample, the present disclosure extends to using phosphors and/orquantum dot technology for this purpose.

Some embodiments describe 2D holographic reconstructions by way ofexample only. In other embodiments, the holographic reconstruction is a3D holographic reconstruction. That is, in some embodiments, eachcomputer-generated hologram forms a 3D holographic reconstruction.

The methods and processes described herein may be embodied on acomputer-readable medium. The term “computer-readable medium” includes amedium arranged to store data temporarily or permanently such asrandom-access memory (RAM), read-only memory (ROM), buffer memory, flashmemory, and cache memory. The term “computer-readable medium” shall alsobe taken to include any medium, or combination of multiple media, thatis capable of storing instructions for execution by a machine such thatthe instructions, when executed by one or more processors, cause themachine to perform any one or more of the methodologies describedherein, in whole or in part.

The term “computer-readable medium” also encompasses cloud-based storagesystems. The term “computer-readable medium” includes, but is notlimited to, one or more tangible and non-transitory data repositories(e.g., data volumes) in the example form of a solid-state memory chip,an optical disc, a magnetic disc, or any suitable combination thereof.In some example embodiments, the instructions for execution may becommunicated by a carrier medium. Examples of such a carrier mediuminclude a transient medium (e.g., a propagating signal that communicatesinstructions).

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thescope of the appended claims. The present disclosure covers allmodifications and variations within the scope of the appended claims andtheir equivalents.

While some embodiments have been illustrated and described in detail inthe appended drawings and the foregoing description, such illustrationand description are to be considered illustrative and not restrictive.Other variations to the disclosed embodiments can be understood andeffected in practicing the claims, from a study of the drawings, thedisclosure, and the appended claims. The mere fact that certain measuresor features are recited in mutually different dependent claims does notindicate that a combination of these measures or features cannot beused. Any reference signs in the claims should not be construed aslimiting the scope.

What is claimed is:
 1. A holographic projector comprising: a spatiallight modulator arranged to display a hologram of a light pattern forprojection and to spatially-modulate light to form a holographicreconstruction, wherein the holographic reconstruction isspatially-separated from the spatial light modulator; a detector arraycomprising a plurality of light detection elements arranged to detectlight corresponding to a respective plurality of positions of theholographic reconstruction and to provide a respective plurality ofoutput signals related to light detection; and a fault detection circuitarranged to compare one or more of the plurality of output signals fromthe respective plurality of light detection elements with one or more ofa plurality of expected signals based on the light distribution of thelight pattern.
 2. The holographic projector as claimed in claim 1wherein the fault detection circuit is arranged to alter or to preventfurther light projection, if it identifies a difference between said oneor more output signals from the respective plurality of detectionelements and the one or more expected signals.
 3. The holographicprojector as claimed in claim 2 wherein the fault detection circuit isarranged to alter or to prevent further light projection only if theidentified difference is greater than an acceptability value.
 4. Theholographic projector as claimed in claim 1 wherein the one or more ofthe plurality of expected signals is time-varying.
 5. The holographicprojector as claimed in claim 1 wherein each light pattern of a sequenceof light patterns for projection is configured such that only onedetection element of the plurality of detection elements should receivelight of the holographic reconstruction at a time.
 6. The holographicprojector as claimed in claim 1 wherein each light pattern of a sequenceof light patterns for projection is configured such that the detectionelement, or the specific combination of detection elements, that shouldreceive light changes with each successive light pattern of the sequenceof light patterns.
 7. The holographic projector as claimed in claim 1wherein the light pattern for projection comprises a primary lightpattern region and a secondary light pattern region, and each positionof the plurality of positions is within the secondary light patternregion.
 8. The holographic projector as claimed in claim 7 wherein thesecondary light pattern region is different to the primary light patternregion.
 9. The holographic projector as claimed in claim 7 wherein thesecond light pattern region is spatially-separated from the primarylight pattern region.
 10. The holographic projector as claimed in claim1 wherein each position of the plurality of positions that arerespectively monitored by the plurality of detection elements is withina higher-order repeat of a zero-order holographic replay field.
 11. Theholographic projector as claimed in claim 10 wherein the plurality ofpositions within a higher-order repeat are substantially adjacent to thezero-order holographic replay field.
 12. The holographic projector asclaimed in claim 1 wherein the light pattern comprises an array of lightspots for light detection and ranging, “LIDAR”.
 13. The holographicprojector as claimed in claim 12 wherein the holographic projectionsystem further comprises a LIDAR controller arranged to move or changethe holographic replay field in time such that each light spot of thearray of light spots effectively occupies a plurality of differentpositions on the holographic replay plane during a scan period, whereinthe movement of the light spots to their different positions during thescan period correlates with the plurality of positions respectivelymonitored by the plurality of detection elements.
 14. A method ofmonitoring operation of a holographic projector, the holographicprojector comprising: a spatial light modulator arranged to display ahologram of a light pattern and to spatially-modulate light to form aholographic reconstruction, wherein the holographic reconstruction isspatially-separated from the spatial light modulator; a detector arraycomprising a plurality of light detection elements arranged to detectlight corresponding to a respective plurality of positions of theholographic reconstruction and to provide a respective plurality ofoutput signals related to light detection; and a fault detectioncircuit; the method comprising: displaying, at the spatial lightmodulator, a hologram of a light pattern; illuminating the spatial lightmodulator, to form a holographic reconstruction of the light pattern,detecting, at the detector array, a light signal corresponding to theholographic reconstruction; receiving, at the fault detection circuit,an output signal from a light detection element, within the detectorarray, relating to the detected light signal corresponding to theholographic reconstruction; and comparing the received output signalwith one or more of a plurality of expected signals, which are based onthe light distribution of the light pattern.
 15. The method of claim 14wherein the fault detection further determines, as a result of saidcomparison, whether any difference exists between the received outputsignal and the one or more of a plurality of expected signals.
 16. Themethod of claim 15, wherein the fault detection further determineswhether a difference, if it exists, is greater than an acceptabilityvalue.
 17. The method of claim 15 further comprising controlling theholographic projector so that, if it is determined that a differenceexists between the received output signal and the one or more of aplurality of expected signals, or if it is determined that a differenceexists that is greater than an acceptability value, further lightprojection is prevented or altered.
 18. The method of any of claim 14,wherein said method is a computer-implemented method.
 19. A computerprogram comprising instructions which, when executed by data processingapparatus, causes the data processing apparatus to perform a methodaccording to claim
 14. 20. A computer readable medium storing a computerprogram according to claim 19.